Well, I finally got around to testing your level again, and I've got to say, it looks great for the most part. It's got some issues though.
First off, I think I should make it clear that this level... is long. Really long. While I have no problem with the effort you put into making such an intricate level of this length, my primary concern is that one midway point just isn't going to cut it, especially when you consider this level throws a good deal of challenge at the player through most of it. My suggestion?
-Move the first Midway Point to Screen 0C in Level 3A. I consider this to be the 1/3rd mark of the level.
-Add a second Midway Point ar Screen 01 of Level 3B, where the path splits. This is about 2/3rds through the level no matter which route you take, and this is where is the level gets to be the most challenging in my opinion. Starting at this point would be very helpful.
Anyways, as for the rest of the issues I had with this level, some screenshots:
It was mentioned before that the Dry Bones was glitchy-looking, but here's screenshot proof.
Don't you think it's a little odd that taking what most players would assume is the normal route brings me to an SMWCoin...
...while this hidden ladder doesn't, and just brings me to the Midway Point at Screen 0D of Level 12D? (Which I believe I mentioned you should relocate)
I still think you should be having these Pirahnas coming out of pipes instead of random areas of ground, especially considering some of them do come out of pipes in your level.
On top of this springboard backtrack being an example of Item Babysitting...
...all I get for it is a powerup. Consider that there was a Fireflower right before where the springboard was needed; why on earth would I want another powerup? In fact, there's a good chance you'd lose a powerup going between those four screens for that springboard and back, so the whole purpose of that powerup is defeated in two ways. I'd recommend just doing away with the powerup and springboard entirely.
This poor Thwimp gets carried into the oil by that conveyor belt.
You shouldn't require the player to be standing right between the correct pixels of these two Trash Blocks (or jump-grabbing them) just to be able to get past this Thwomp unscathed. Though I suppose it wouldn't be a problem if you just removed those spikes.
Another ridiculous-to-obtain powerup, this one basically requires jumping off a Parakoopa at the right position, while holding something to actually hit the block mind you.
I also noticed you used the ledge-style conveyors normally found in SMW by default. I'll address that in a bit.
Was that Disco Shell supposed to come bowling towards the Thwomp like this?
Finally, one more thing I forgot to bring up, is that you were using the "default" conveyor tiles throughout the level as opposed to the solid blocks that are currently in the Base ROM. While I can't say for sure this was how you wanted to build your level or not, I should point out that it causes some oddities to happen...
...like this. Another reason you might want to use the solid conveyor blocks is because other levels in the hack are also using them (including my first level, Rusted Retribution, which I recently added them to), so it seems a little odd that conveyors would function differently here.
Other than all that, I'd say you've done a great job with the level. There was slowdown in some spots, but hopefully that's nothing FastROM can't handle.