Here's the promised review, bro.
First thing I'd like to point out is that your hack will likely break on real hardware, since you've used the old Romi's Addmusic. In songs that use echo, I could hear clicks/noise before the song started playing while playing in higan. As for the hack itself, it's pretty basic, yet still very fun to play. It also made me happy to see how you worried about fixing cutoff by merging GFX, as seen with slopes and pipes, but also columns and lava, the goal post and donut bridge railings. Now for the little details that warrant revision.
> Pretty minor and optional, but the lava animation is desynced where the column is placed.
This also applies to palette animation of this level.
> Also minor, but powerups are having priority over brown blocks.
Either tweak Ice Man's HDMA patch that you've probably used, or add priority to brown blocks.
> If I eat a blue or yellow Parakoopa, and spit them, they act weirdly.
Either remove Yoshi from the level or remove the custom Paratroopas.
> (using your own pic I haven't noticed before lol) Weird tree placement where the pipe is located.
Move or remove it. Also other hint: try adding more terrain variety to your overworld, putting some more little hills here and there to spice stuff up.
> Koopas have inconsistent frames, especially shelless ones, as they flip from vanilla to the alternate SMW style.
> The Green Switch level was kinda pointless, because as soon as I entered it, I basically went starightforward screens and pressed the switch. Other than that, there's a pretty simple, optional puzzle to do in order to get an 1up. I recommend making this level somewhat challenging instead of bonus-like, though do as you wish.
> It's tricky to backtrack carrying a P-switch in Platforms of Mush to get the secret exit, especially where a vine is located. Try revising this level and tweak a bit the offending section.
> The layer 3 cloud effect on Land's End is pretty neat, but it obstructs the player's vision. I took several hits from enemies because of that. Perhaps make it translucent?
> The already mentioned Addmusic4 issue. It's likely most accurate emulators will crash at some point. Switch to AddmusicK, as it's a requirement for today's hacks to be accepted in this site.
Aside from these things, it was a cool demo. I loved the gimmick of Wet-Dry Cave, and liked Land's End's cloud effect as well, even though it made the level a bit hard to see. I also liked the intro cutscene with the little event added in there.
Keep going, and good luck with this hack.