Nice level you got there. I really like the gimmick.
There are a few small things, though:
The stone block above the donut block is too low, jumping on the donut block can be tricky. You can also glitch through the line-guide platform when you're squashed between the roof and the platform.
I liked the trick with the key, but I believe this collab doesn't use key exits. You could place a locked door further ahead with a star behind it.
When you kill an enemy right after the midpoint, there is one piece of lave that doesn't dissappear.
The same sort of thing happened later in the bowser room (which was creative, to say the least). When you kill bowser with fireballs, the bridge doesn't dissappear.
Also, there was an excessive amount of feathers in the first part, maybe you should delete some.
I like what you have so far, keep up the good work!