I was able to get into the level 2 room, due to the floaty jump as you noted, I wasn't able to complete it. You have to shrink yourself right down (keep pressing R until you can't any more) and then jump on the right-side of the opening. For some reason (I suspect a misplaced collider), I could only seem to get through the right-hand side of the opening, not the left (and possibly not the middle either).