Well, it took me more than a thousand re-reads, and several analysis periods, carefully contemplating on what It did in the book and what It should have done instead, this is where my conclusion comes from
Believe you me, I considered this reply very carefully, since as you skillfully pointed out, no person can understand or comprehend It's full capabilities, therefore a wrong assumption can very easily lead the person's death, who gets attacked by It.
I think the problem is that you are considering the creature scientifically, as if it is some animal following the evolutionary trail. You are looking for cause and effect based on what we see it do (and what we do not see) and applying limitations based on these observations. If we were studying a physical creature that is driven by physical needs and mortal considerations, that would be the most logical way to approach things. The monster that has come to "haunt" Derry is none of those things.
You're right about that a clown is a preferred form of It, and It utilizes it mostly when on "neutral grounds", after all, It is after kids, and even the book mentions, which kid would not like a clown (actually, there are precedents that several didn't). As for the actual monster outfits, It needs to read the victim's mind, but that monster will be the that person's very own horror image until the person actually shouts it out loudly what is it (this is what happened at the 29 Neibolt Street showdown). Richie shouted "It's the Teenager Werewolf", and It's shape solidified into the Teenager Werewolf for everyone else, so it became a common monster. Uh, same goes for the Bird. Mike saw it once, told the others, and the Bird actually appeared in the sewers. As for the first Neibolt Street attack, Bill saw a clown-faced devil (according to the book), whilst Richie saw It as a werewolf, since Bill did not see the matinee movie.
Again, I think you are confusing preferred method of attack with only method. The monster "likes" to take a form of fear from the victim's mind so that it can season the meat. It doesn't have to do this. The creature is quite capable of taking a victim any number of ways. Sometimes it just "rides" a person and murders them in their sleep. Are you forgetting that it was going to torture, rape, and murder Beverly using her father as a vessel? Are you forgetting that it was going to use a guy working at the hospital to murder Mike (in his sleep no less)? The creature with a thousand faces also has a thousand methods. It has vast powers far beyond what we see day to day. They are mentioned in the book. They are implied all the time. Your question appears to be why doesn't it use them more effectively? We will come back to this later. I don't want to go too far on a tangent from what I am directly answering in the quote above. The monster clearly prefers to attack an individual (alone) when it can because getting the perfect form that will scare everyone is going to be more challenging. What scares one person is unlikely to scare another. We see this problem played out when the Losers battle It in the sewers, long before they reach its lair. It doesn't have to solidify into one monster for the group. If you will remember, Richie saw the werewolf chasing them when Bill and Richie were on the bike. Bill saw the clown. An interesting question which we don't know the answer to is what would have happened if it had caught them. I suspect that Richie could have been clawed by werewolf claws, but Bill would have been bludgeoned or somehow mauled in an appropriate way to how he perceived it. This is another tangent (but an important one) we will come back to later. The only thing that is truly important about the Losers coming to agreement on what it looks like is that they are all on the same page of what it is at that moment, what it can do, and more importantly what can hurt it. I think the monster would have been perfectly content for all of them to continue to see it differently, frozen by their own personal terrors, while it killed them. What is important to understand is that it didn't settle on the werewolf, that form was impressed upon it by the Losers. By opening itself up to use their personal fears against them, it made itself vulnerable to being molded by them. That doesn't mean it couldn't have simply chosen something for itself from its portfolio. Of course it could have... but up until that point... it has no reason to truly fear them. It simply acted as it always acts because why not?
As for the manifestations, they are real enough to cause harm, but I believe they're not actual living parts of It. Popping It's balloons for instance, would not cause any harm to It. Henry's switchblade, for instance. As for the Plymouth Fury, that may or may not have been real. If it was a manifestation, I can understand why it worked - It can take the information out of anyone's mind, so once he probes the mind of a car mechanic or someone who knows what a car is made of, how does it work and such, then It could manifest one, though there might be limits to this ability. But I am 100% sure It could not transform itself to a car, or any other machine for that matter, because of the moving parts, battery, fuel, and pretty much everything else which needs a car to work. It could probably mimic the appearance of machines, but it would still be living tissue having only the looks of solid metal. If someone would be scared of the Terminator, or a nasty Strogg creature from Quake 4, then It could probably assume its form, however, since It is a living creature as it is, it cannot become metal, steel, stone or anything else which does not have life in it. The second reason is that machines or non-living things are generally not scarry, they cannot really be used to terrify people, so It would see no reason to shapeshift into one anyway.
Every part of It is alive. Don't forget that even its saliva squirmed away in the sewer. This is a spiritual entity first, and only physical when it imposes itself on our reality so that it can affect physical things. The monster is a ghost, spirit, god, demon, or simply animus if you like. It has no form in the real world without expending effort to do so. It cannot affect the real world at all unless it takes a physical form or possesses someone appropriate and uses them. The trappings of the monster lead even us, the reader, astray from its true nature. It is a hungry ghost. Let us look to a similar spirit in the works of Tolkien, i.e. Ungoliant. She too was a cosmic entity, a spiritual creature who chose to take physical form. Her only desire was to eat. That was the extent of her ambition, i.e. to feed her personal hunger. The major difference appears to be that in Middle Earth the decision to go back and forth between flesh and spirit is more permanent. Ungoliant took physical form because that was what was required to eat. The same is true of Pennywise. Every illusion the monster creates, everything it spawns into the world, and each individual thing is part of itself and thus alive. The key is that the manifested parts of itself are bound to the laws of the manifestation. If it manifests itself as flying leeches, those creatures will simply have the ability (mental and physical) of the same. Let us not forget Hockstetter, who is driven by one part of It into another. When you talk about the creature being able to mimic a car, you are missing the point. It is't a mimic. It becomes a car. It becomes a vampire. It becomes flying leeches. Perception is reality. And for the last time, it doesn't HAVE to become something scary. That is merely a choice. We have seen it choose to be enticing countless times, to lure its prey closer.
This intelligence may very well apply to living creatures with special methods of attack. If It would shapeshift into Medusa, people could not be turned to stone simply by looking at Its eyes. Also, It is completely devoid of any projectile attack methods, has only melee attacks. So if It would shapeshift into...um...an Imp from Doom 3, it would be scarry as hell, could scratch and bite, but could not throw fireballs at all. Technically as long as the victim keeps its distance, he'd be safe from It.
It could do all of these things, but quite often that would defeat the point of the process. It is rarely looking for a clean, quick kill. It is trying to cook its meal and season it to taste. That process requires time and application. For adults it is too much work for a lazy creature to bother with. That doesn't mean it can't do it. It means it doesn't want to exert the effort. Therein lies a clue. It takes effort to shapeshift. That is why it often slips into the form of the clown, its default neutral. It takes effort to peer into a mind and pull out that fear. It takes effort to manifest it. It takes even more effort to manifest for lots of people at the same time. We see a clue in that again, supporting this notion, in why it prefers a one on one hunt, and why it will often blow off a hunt if/when the prey reaches a populated area. The creature is lazy and doesn't like to expend too much energy. We see implication in this too, i.e. why does it go to sleep? The long and the short of it is that the monster is feeding for a reason (or several) and hunting expends energy. You want to spend the least energy hunting you can, so that you profit energy in a successful hunt. It is actually quite logical. The seasoning of the meat, so to speak, is clearly important to the monster. The prey might just taste better, or perhaps that gives a bigger blast of power. The specifics aren't important here. What is important is that the monster clearly prefers to terrorize prey before eating it. Turning the victim off like a light is counterproductive to that. It only tries to kill that way when there is a need, and when it kills that way (i.e. not for feeding) it tends to use other agents or possessed dogsbodies.
As for the second paragraph, you're mostly right, It didn't know these boys are going to team up against It for the kill, so It could allow the luxury to let'em escape. It just seemed strange in contrast to when It actually acts smart and tactical, trying to outwit the victim instead of scaring the living sht out of him. And when the victim is sleeping, there is no image to project, so It simply cannot materialize itself into any shape. Without shape, it cannot kill anything. This is clearly reinforced when Eddie Corcoran dies, It immediately loses its shape. In Patrick's case, however, Beverly was in the close proximity, and could witness the winged leeches, so they remained there, and even attacked Bev until she finally left. When all the Losers have returned, there was no one to scare, so the remaining leeches lost their shape, and reunited with It.
Again, you are attributing too much knowledge to it, and a tactical approach you and I would apply after years of watching horror movies.
You are clearly a very objective motivated person. But let's address Hockstetter and Bev. As I pointed out before, it truly becomes whatever form it takes. If it has already taken the form, there is no glamour effect. The slugs were already in the refrigerator. It had prepared a trap for Hockstetter. Beverly saw what Hockstetter saw. It is unlikely she shared the same fear as him. That means his manifestation, exactly, was visible to her. Those damn things were real. They were part of It, but quite real. They were a trap it set, solidified part of itself into reality (which is why Bev saw them the same... felt them in fact) and drove Hockstetter into another manifestation of It which was more "flexible" shall we say?
As to Eddie Corcoran, it doesn't "lose" its shape... it just doesn't need it anymore. It drops the glamour because that is effort. Why spend energy you no longer need to spend? The evidence doesn't show the creature is forced to do anything it doesn't want to do. The only exceptions is when it is choosing to shapeshift and tap the minds of its prey. That potent power is also its weakness, if the prey figures out it can take advantage of it. Crosses don't hurt vampires. It is the belief in crosses that hurts vampires.
So let's get back to some of those asides from earlier. If we assume the creature is supernatural, i.e. a powerful spirit called the Deadlights from outside reality that has found a way through the cosmic barrier (either finding a pinhole or puncturing a small one), then we must accept that the creature known sometimes as Pennywise is a bit of the Deadlights oozing through that hole, a tendril of that thing's power. As spirit, it is limited to spirit. It is a corruption, unnatural, and simply wrong. It is from outside. It is a hungry ghost that has limited options. It can simply lurk and watch, perhaps influencing or possessing those few whose nature allows it, or it can become physical to feed its urges. To become physical is also to become vulnerable. It cannot affect the physical world without itself becoming somewhat vulnerable to it. The insane Deadlights are also alien to our world. It has no concept of form or function. It must, therefore, refract its evil light through the prism of our minds to take shape. The tendril, an Avatar if you will, is more logical and thinking than the Deadlights because that too is part of our reality. It isn't just the body of the creature that is refracted and made manifest in the image of our world, it is also the monster's mind. Here it can think. Beyond the barrier it is an endless babbling madness. The intellect, i.e. the very personality, of Pennywise came to be also drawing upon our world. It, like us, is a product of the time and place. This also means that in the unlikely event of its destruction, Pennywise is truly dead. Another tendril that manages to get through somehow would be a product of its time and place. Of course, the implication of the book is that the death of Pennywise has sealed off that level of the Tower to the Deadlights. The pinhole was sealed when the tendril is cut off so to speak.
But let's get cosmic shall we? Let's try to paint a physical picture of this to conceptualize it. Imagine the Deadlights outside of everything. Imagine that cosmic barrier between reality and whatever "outside" actually means. That hideous madness of lights wants in badly. Like an octopus trying to get at a fish inside a bottle, it is always gibbering and feeling its way around the cosmic barrier trying to find a way in. It finds (or makes) a tiny hole in the barrier through which its light shines. Can all of the Deadlights get through that hole? No. But now there is a beam of light shining across the Macroverse. Visualize that orange light, the barrier, and the beam of orange light coming through it. That light crosses the vast distance and BLAMMO comes out of the sky and blasts a hole on planet earth. That was just the initial impact so to speak. Now think about that light still beaming there. The initial damage is done, but that unnatural light coming through the pinhole from beyond is still bathing Derry. It is a flickering light to be sure, as beyond all reality the Deadlights seethe and churn and constantly seeking other ways in. Think of it as a pattern as the Deadlights move around doing "other" things. Sometimes the light burns bright, a solid beam if you will, on Derry every 27-30+ years or so. The rest of the time it is a flickering, faint light as the light of evil has moved around. The Avatar that is Pennywise exists in that light and is strongest (awake) when it is beaming. It sleeps the rest of the time. The light never fails entirely though, which is why the corruption and effect on Derry is always present. Think of that beam as a pathway across which this corruption and madness flows. It is a path that can go both ways, and does in fact like the end of an evil rainbow. The Losers found the end of that rainbow under Derry, i.e. the point of impact of the beam, where the thinny rests. One end is here; the other end is the Deadlights. Kill Pennywise and the tendril is severed or snaps back, and closing that pinhole in the barrier. The thinny remains as the barrier between this level of the Tower and the macroverse is thin now, but the cosmic barrier is another thing altogether. Out there the Deadlights seethe and pulse, circling the barrier to find other ways in.