Mirrors: "you can finish the game on a single mirror time slot," Now this gave me the idea I shared in the Discord. I think you could make it so that there is a checkpoint at the skull bench and when you arrive, it's a checkpoint and then every time you drop off a skull it's a checkpoint. Then you could drastically reduce the time that the Mirrors will save you at death, and if the tie runs out before the player dies (hopefully it typically runs out on their way back through the challenge a second time to return to the mirror) then they'll die and lose any gems and such that they had acquired since the last skull was placed. You could expand this idea to a full game as well, but I'd have to see how the game is formatted to be able to give additional help for that.
Collisions: "[...] probably should be trimmed [...] some issues detecting collisions on ladders" I think you should do a hidden player mask that is larger for the ladders as there was multiple times that I was unable to go up or go down them when I expected to grab them easily. Though I think it was you who was saying in the jam discord that the trouble with ladders is that everyone expects them to "just work" but everyone expects them to work differently. I am definitely in agreement with that sentiment, even if it wasn't you that said that. Then you could make the player hitbox smaller without issue.
Jump style: "I wanted something that was closer to real jump physics" I figured this was the case, and I respect your decision to do it this way. that's absolutely a core part of the experience and if it was different, the jumps would've been designed differently. I'm partial to games like Hollow knight where i have a lot of control over the jumps, so that's part of what's going on here. It is a bit weird that my game is the opposite. you can move left and right while jumping with exact same speed and control as when you're on the ground BUT your jump height is fixed. isn't that funny?
Map/level design: Loving this part of your response. "pico8 base constrains did not allow for this." It's interesting that in the next section you say you're going to stay within pico 8 after reading this. "going multicart" Do you mean like multiple different Pico 8 game carts?
The future: absolutely ambitious and I love it. Don't be afraid to put it down for later to go and work on other stuff. Coming back to an idea is always an option :)