I believe that it was all an accident. in most of the first half of the series, thats what he syays. and what everyone else says. and I believe him. I think that once he adapted to his new life somewhat, it allowed applegate to add this doubt. as he was in the bigenning of the series, I don not think he would trap himself.
and, in book 43, he looks DOWN and sees the place where he was trapped. not up. he wasn't way off the the side or up on a rafter, I think he was actually between some boxes a few feet from the pools edge or something. it is reasonable to say that there are places to be trapped in the pool complex, even when you're that small.
of course, some things he says might be considered admitting to the fact that he did it on purpose, but I don't think that even THAT means it was. he was under pressure, he had heard the same story for a while, he could have been starting to accept something that wasn't true in the first place. he had time to think over those dozens of books. time to get it wrong. what he thought at the moment of the event, what he told others, is what he really thought, and what he really meant to say. and when he talks, I listen.