That's actually an amazing idea, I can't believe it didn't occur to me to have a special case like that.
Viewing post in small graphical inconsistency (almost fixed)
Stumbled across a few more regarding Chibie. First, her wings don't get shadowed/inked in the Unfinished Swan room. Second, her wing animation freezes if you cat-grab onto a wall while floating (so if they're in the middle of closing when you grab the wall, they'll stay in that partially-open state until you detach). Third, even when gore is disabled, the healing effect when she kills an enemy is still blood-red (though I'm not sure how much you'd count that one). I'd upload some more screen captures to show these, but itch is breaking my images again.
This has been bothering me for a while, but I think I finally know how to voice my concerns: the frame of the spinning spike log that hurts you doesn't look like it should hurt you. This is because it has the bottom of the spike outline bending downward (by a lot!), making it seem as if the spike is pointing towards the camera and the player would land behind it, but that's not the case. I think you were going for one of those slightly-downward-tilted-camera perspectives like what NES Ninja Gaiden does, but the problem is that the entire rest of the game is strictly straight-side-view like the Super Mario games, and this perspective conflict makes that hazard unintuitive. Please redraw the log's harmful frame so that the spike graphic isn't cutting into the log graphic, or at least so it only has the spike's outline overlapping the top pixels of the log. Maybe also make it so the harmful frame has a brighter spike for added clarity.
Noticed this one several days ago but only just now got around to capturing footage: chasing enemies (such as the Human Rockets) will continue gaining momentum while the player is switching/losing powers, allowing them to make much sharper turns than they otherwise could:

EDIT: On the subject of issues I noticed days ago but never got around to mentioning: holding a mega buster charge and entering a door will result in the "eeeeee" charge sound playing continuously, even if you exit to the world map or even the title screen. Even if you then set "SFX Volume" to 0%, the "eeeee" keeps playing. The only way to stop it at that point is to do something that mutes all audio, like die or close the game.
You added the option to turn gore off for those who are squeamish, right? If so, it just occurred to me that you might also wanna add alternatives for stuff like the meat factory and flesh tilesets/backgrounds as well, along with alternatives for the decorations in the fake Reinforcements boss room.
An addendum on the "add obstacles instead" option not being indicated enough: when you go to change the in-game description to mention that it can be either extra platforms OR extra obstacles, please replace "no help" with something less specific like "no change" to make it extra clear that the option isn't just for more help.
Stumbled across a new one just now: if you rocket-boost into a concave ceiling corner in just the right way, you can get stuck forever.

EDIT: Oh, and for completeness' sake, I figured I'd also mention this other, less severe glitch I encountered while trying to recreate the previous one: rocket-boosting into a convex ceiling corner can cause the rocket to hang, but this one is much easier to escape from.

Here's another one I've known about for a while but didn't get around to reporting until now: the Fire Bomb Girls' projectiles don't disappear when off-screened. Instead, they hang in limbo until they're back in the screen's activation zone, potentially resulting in an unexpected cheap hit.

(Note: I don't mean the first two stray ones that actually hit me, but the ones after I start jumping back up.)
Falling into a bottomless pit in a dark level still has the player's spotlight bounce back into view for a bit. I figure you already knew about this one and tried to mask it by covering the dark-level pits with death water, but the problem with masking an issue instead of fixing it is that if you miss a spot, well...

Oh, and that reminds me: if the player activates a solid donut block, it always comes back as a regular jump-through donut block. Was that intentional? If so, what's the point of having solid donut blocks in the first place if it makes no difference whether or not they stay that way? Was it just for visual variety?
If I remember correctly it's to prevent a softlock somewhere (in DDG1 donuts simply doesn't come back, but they need to in DDG2 because of the multiplayer component, so every section where you can use donuts to descend into a lower area needs to be escapable from below too and solid donuts turning into regular donuts guarantees that)
You mean donuts don't come back. :P "Does" is only for third-person singular nouns in present tense, and "donuts" is plural.
I will say, I'm having trouble seeing how players can get stuck in multiplayer without getting stuck in that same spot in single player, but I guess I'll take your word for it.
Anyway, here's another that I noticed a few days ago or so: the diagonal whip has a much-thinner hitbox than the horizontal whip. This is easiest to notice while trying to clear away the green skeletons' fog from a standstill, which is significantly harder to do diagonally:

You can tell by the skeleton's throwing animation that the top image isn't the fog disappearing on its own because of when those animations happen relative to each other compared to when they happen in the bottom image, where the fog *does* disappear on its own.
A bit more feedback: I like how the powerup-loss effect happens when the player is only one hit away from death (or two hits for certain weak enemy attacks or high-HP characters), as it functions as an in-your-face warning that you're about to die and should be more careful. However, there's no such warning during boss fights, where the player is immune to losing powerups; it's just the same as every other taking-damage animation until suddenly you're dead. The most that happens is that your tiny health bar in the distant corner of the screen subtly glows red. It'd be nice if you could add some type of more-obvious low-HP warning for those situations as well.
I only just now discovered that Holy Cow's illumination orb also doubles as an attack that can hurt enemies (I was under the impression that it was ONLY for "lighting the way" and the sky lasers were her only offensive spell), so whenever you go to reword their descriptions like I suggested before, I'd appreciate if you made this detail clearer as well.