Thanks for all the feedback!
I ended up scrapping the head-dropping mechanic from the snowmen, because things quickly ended up crowded and it didn't turn out to be too fun to deal with. I did have them vulnerable to fireballs at the beginning, but that ended up making them too easy, so now they're fireproof.
I've left this thread alone for longer than I had planned, mostly because I had various other small stuff to do and it took me way too long to get back into the swing of things.
Since my last post here, I've actually finished world 4-1, and made some progress on the visuals of world 4-2! This is going to be a frozen lake kind of level - with the most ambitious background effect to date:
The ground stretches and shrinks depending on your vertical position, and splits into an above-ground section and an underwater section. I think it's come out pretty neat, though it's ridiculous how much work (and how many failed attempts) went into it. It's not even 100% done yet, it's just barely in a showable state.
The sad news is that this effect is so heavy on the resources that it needs
SA-1 to actually not break or lag beyond belief. I am planning to port to SA-1, but only after the rest of the hack is done, so I can't actually use this effect in my hack yet. (the GIF is from a second ROM I set up just for that.) I'll see what to do about that.