This was quite entertaining, once I got past that bug. That one unique song was superb btw. Too bad there wasn't a full compliment of themes to enjoy.
The concept of this mod works great in Cave Story. Have you been playing Celeste?
As for the difficulty... it was mostly lacking in challenge until the last two checkpoints or so. You introduce some interesting mechanics, but only touch on them or use them in a tiny area. Like water. There was no threat in drowning, so it fails to live up to its potential. Or the air sparkles. Introduced and used, what was it? Once? Neat, but in this time of game you usually introduce new concepts and expand upon what you've learned challenging you to refine your skills in new ways. Oh and the excess amount of HP... I almost think you should have multiple difficulties, but just a variation on how much health you start with. These are just my thoughts though...
I really was happy to play this when I got past that bug xD I started to write a post saying "1/10 I really wanted to climb something". Glad I opened it up again and realized my mistake xD
PS... Time attack looks to be a fun challenge! (Especially thanks to bonus stage 1...)
I currently have a snail time of 9'06"6 on the main game.