Also, paraphrasing
my post on NESDev, I was wondering what else could you get out of bounds.
I cheated to avoid death (address $040A is the bubble state apparently) and have infinite double jumps ($003D is the flag for it), being able to explore a lot of glitched rooms.
These are the rooms I could reach, the missing ones crash the game, probably the resulting tile data pointer points to something that break the game or loop indefinitely. An interesting result is that the whole map loops, there on the right you can see a copy of the normal area but 1 screen up. Another cool thing is that blue room in the bottom to the right, for some reason there's a copy of the Bubble, you end up controlling both.
My whole idea was to reach the credits room earlier by abusing these rooms, but as you can see you can't get there unfortunately. Also i could move only with cheats, so many random solid tiles it's hard to maneuver there.