First of all, let me start by thanking you for the review.
I'm pretty sure the music won't break, seeing as I've tested most of the hack in the latest version of bsnes. Works just fine (save for the little crackling, which is really just an annoyance). It still uses HuFlungDu's Addmusic (which is supposed to work on real hardware). The only reason I haven't switched to AddmusicK is that every single time I decide to do it, I either find a new patch which crashes the game (e.g. the Leave Yoshi patch) or doesn't work with it (granted, I could probably fix it) or something odd that it does (such as the jumping SFX which really irks me for whatever reason). Granted, the newer versions of it work with the VWF dialogues (as proven by this
monstrosity that I created just to test them) and that's what's most important for me. Now to address every issue you mentioned individually.
The uneven columns are a fairly new addition to castles and frankly, I know very well the lava desyncs and I've got no idea why I haven't fixed it. I definitely will. I'll be honest, the palette animation in Wet-Dry Cave was an accident (it uses consecutive ExAnimation slots, which is what causes this; there's information about it in the help file), however I ended up liking how it looks and decided to keep it (to me it looks like the water is flowing in).
The powerup priority is a known problem of the HDMA patch. I'd rather not add priority to the brown blocks, but I didn't know you could change the patch to fix it. I'll look into it.
Ah, yes, the custom parakoopas. They're really buggy and I'm way too scared to touch their code. Weird Yoshi interaction is definitely a thing with them (along with their weird hitbox, bad ground interaction, not being flammable and probably other things). I might make them inedible, as I really want to keep both them and Yoshi (worst case scenario, Yoshi's going, considering the level more or less revolves around them).
I agree, the pipe is weird. There are unused reveal tiles on the overworld, I'll make it hidden by default.
The Koopas are a mix between vanilla and alternate graphics. Their shells use the alternate graphics, whereas their bodies don't (I don't like how they look). I thought I'd made all shelless Koopas use their vanilla graphics (looking at Lunar Magic, the sliding one is the only one I've missed). I'm not sure what you mean.
My main goal with the switch levels is to make the puzzles optional. The Yellow Switch (technically not in the demo) is a little more complicated and I might make the later palaces harder.
I was playing through the demo this morning and I thought the same thing about Platforms of Mush. The switch is basically useless item babysitting. It's going away, that's for sure.
Land's End is really the place where the difficulty peaks in World 1 and the cloud effect is actually intended to obscure your view. In most places the clouds are way lower down, so all you really need to do is wait if you're uncertain about a jump or an enemy. It seemed a little unfair at first, but then I actually liked the idea of it.
Thanks a ton for all the feedback. It's really helpful and I'll take everything you said into consideration.