First, I have a couple of pics I want to show:
Eh, now what do I do? Ok, my initial reaction was to use the springboard to get this P-switch. While trying, desperately - and failing miserably, to get back out, I managed to get myself stuck up there. Welp! Game reset for me... Yea, I would remove that first springboard. What is it supposed to be used for anyway? It wasn't until my third attempt that I realized I should be using the throw block to obtain the P-switch. Otherwise, that springboard is a pretty dirty trick.
Ok, I know the diggable sand blocks aren't in the level yet, and that will likely solve this issue once they're applied (if?), but this Chuck appears to be performing some rigorous training for the upcoming season by running in place continuously.
Moving on, I'm a bit confused over the diggable sand concept. Like Axem pointed out, the gimmick is damn near absent other than the very beginning of the level. If you're going to use diggable sand; make full use of it, otherwise scrap the gimmick. Don't rely on one section where you're forced to use it to continue on and then tiny pockets here and there that serve no real purpose. Of course, this is assuming you are still using the concept since your original IPS post made mention of the gimmick but your latest post hadn't, so...I don't know.
If you do continue with the gimmick, again as Axem suggested, replace the giant ladder with diggable sand. It could also be used to obtain the final SMWC coin, replacing the first bridged pit on screen 14 with a giant shaft of sand, or it could even replace the falling platforms on screen 15. There's definitely some areas in the level where it could benefit from the gimmick if it's used.
Regarding springboards: why are there so many of them? They're everywhere! I think you're relying on them way to heavily, and some of them can come across as redundant since you can just boo-jump, such as the ones on screens 06 and 1A. To me, the one on 12 is the only one that serves a real purpose since it can be used to find a hidden 1up. Even the ones on screen 14 could be replaced with something, ideally some kind of goundwork so you can open up a couple sprite slots for something dangerous besides the bullet generator.
The second half of the level seems incredibly empty compared to the first half, and it's not just the lack of spritework (again, completely relying on the bullet generator) or the wide open spaces - there's something else about it that just feels...bleh. Maybe it's the total lack of coins or the sudden loss of interesting architecture (especially screens 18+)? Hmm, I can't really place my finger on it.
As far as what I liked: I think the architecture in the first half (up through screen 0C) isn't all that bad. Other than a few iffy sections, such as the ladder setups on screens 04/05 or the upper paths on screen 08-0B, the design has a nice, varied approach. However, the spritework doesn't quite gel, be it with the architecture or with other sprites. For example, I only ever felt threatened by the boos in the first half of the level while the bullet bills were nothing more than harmless onlookers, while the exact opposite held true for the second half of the level.
Oh, and two final tidbits:
The coin trail on screen 1A leads right into a boo. Move that boo over to the right. And make sure any blocks above a slope are set to act like tile 100 instead of 130. Mario has trouble getting up slopes otherwise.