"Sometimes he would lie in the dark and think he heard chuckling noises coming from the drains and he would think: You're going nuts, old-timer."
This is during one of Ev Hillman's scenes. I don't remember anyone seeing Pennwyise coming out of the drain. I'm not positive of it, but unless someone points directly to the supposed scene, I'm going to keep assuming that my memory is the correct one.
As for the question of whether Derry would exist on the level of the Tower where King exists...the way I see that, it's possible that this might be a different level than the one the version of King in Song of Susannah lived on. So perhaps there are levels in which King is real and Derry is a real place, too.
It's a good, fair question, though.