I never really saw It as insane at all
there are a couple of insane characters in the book - Butch, Henry, Patrick, and a lot more - but It doesn't show signs of insanity like them. In fact, It even states itself that its brain embraces the whole America, being so smart. After all, It can collect information and knowledge from anyone's brain, and even control weak-willed people. I'd say It is chaotic as hell, but completely sane and cognizant. This is why I'm so surprised when It makes seemingly illogical decisions and seems to sabotage Its own efforts.
Most of It's actions indeed has a meaning and a reason, this is why it was surprising to see why It land a serious blow on Derry whenever It awakens or goes to sleep, since I couldn't even imagine otherwordly purposes for this
and frankly, this is also part of It's nature, so understanding it better certainly serves everyone good
When It awakes and something REALLY bad happens, that sets off the alarm at people, and possibly hastens bringing the curfew in force again. This is why I found this habit of It illogical - basically this makes Its task harder, since during the curfew, there's less victims to pick from. I think Bill mentions that the curfew worked wonders back then. Of course, if It is contented with less victims, then there is no big deal.
You also brought up an interesting topic - what if It's existence and whereabouts would be revealed for the entire city? I don't think people would just evacuate, especially Americans. Usually, when a strange otherworldly creature's presence gets known by "da gubbmint", it'd flood the entire city with soldiers armed to the teeth, explore the whole sewer system, eventually find It, and mince it. And then the scientists would follow, taking samples, and the FBI telling everyone to shut up, and be thankful we got rid of that monster. I know it's cliché and stereotypical, but that'd happen exactly if It would be discovered by the authorities. And what resistance It could offer? Seriously. It may scare, kill or mind-control some people some of the time, but It could not kill ALL the people ALL the time. Some soldiers would be massacred by It, some others would be lost in the sewers, but eventually the sheer numbers of the US Army would overpower It for good. Even if It would unleash all its tricks, it's only a matter of time when It gets enough wounds to die.
I'm glad you liked our detailed analysis on the creature, Brooks
Now, if It decides to turn up somewhere else again, we'll have the proper knowledge to annihilate it
As for Frederick Cowan's demise, It didn't take him with itself, just killed him. Had no time, as his mother was already running up the stairs, she even heard Its laughter (and said it sounded non-human). A hole being too small is not a problem for It, since between shapeshifts, It is an amorphous silverly-orange-ish goo, and much like the Blob in the 1985 movie, in this form, it has no problems of moving thru small pipes.