Well...my guess is, Al Marsh was not actually It, but rather mind-controlled by It to do its dirty work. And this time, It was really thinking strategically - instead of eerie and scarry monsters, It decided on a more sophiscated, tactical approach, knowing that Beverly will not harm her own father. And since Beverly was the sharpshooter of the Losers, taking her out one way or another would actually serve a strategic purpose rather than just satisfy Its own bloodlust and intimidating the other Losers. I believe the showdown in 29 Neibolt Street was the turning point of the war - after getting some silver right in the eye, It finally learnt that it is actually in great danger, and instead of scaring the living sht out of them, it should try something sly and cunning instead..."brains, not brawn". So It left all monster-costumes at home, and utilized its mind-controlling ability. Seeing how Al Marsh was only a caretaker, a short temper, and almost a complete no-brainer IMHO, It never had a hard time with such weak-minded person.
Now that I got out of the way, I believe It either ceased its mind-controlling on Al, and focused on Victor and Belchy instead, and Al Marsh would probably act like Claude Heroux after the Silver Dollar massacre - puzzled and dozy, like awakening from a coma. He probably didn't even remember a thing. And if that was not the case, the Losers' victory forced It to retreat, and all Its' influences faded away, leaving Al Marsh somewhere in the city in the said condition. I believe when It's mind-control ceased, he'd not lay a finger on Bev after she came home.