Finally 100%ed the game. As I said before, it's really good, but there's one more issue that should be fixed: some vertical sections make the player bounce upward on rocks/spikeballs into another row of rocks/spikeballs (like the tree-trunk level in world 2 or the wrap-around segment in world 5), but since they don't start moving until coming into the screen, there's a chance they'll spawn on top of each other, meaning the player can't get up without taking damage. There's an extra problem in the world 2 level, since waiting for them to come back around after missing one jump causes them to go offscreen and stop moving, so when going over to see what happened, all the spikeballs end up on top of each other:
EDIT: An easy fix for this issue is to shorten the horizontal halls the spikeballs are in, so that said halls are too short to go off-screen.