I have played it, and I found a lot of errors that heavily affect the level design.
I know you said that you are going to fix the OW for the full version, but it looks really bland right now.
That brown block I have hit here is an invisible coin block (and it's not a dotted one). I think you could use dotted invisible coin/note blocks to make them not look tricky to find. Or rather, you could use these you used in the castle.
I know everyone had noted it already, but floating and stacked munchers are a no-no. Also, you could add a bit of a challenge while waiting for the P-switch timer to go out.
This segment is empty without sprites to try to stop you from eventually arriving to the next P-switch. Also, not pictured, but there are cutoff with the note blocks in the goal sequence.
What is the point of the goombas when you could skip a section of the level using them? Also this room of the level is not good.
This segment is also really empty.
This kind of falling segment is really annoying, because some parts that are like "choose the right way or die!" are way boring.
If you really want to keep these Yoshi blocks, you could use another MAP16 block to not look like Mario is touching an invisible wall from the right side.
The red square looks really weird IMO.
If you really wanted to use the mushroom to break the turnblocks, you could make a way so Yoshi can't take the springboard easily.
You can Yoshi jump through the Yoshi blocks, so, I suggest planning around that.
Any reason of the empty BG there?
And also for the coin here?
Fake invisible coin blocks, like the one above me, are not good.
I don't think these signs are really necessary.
Yeah, you need to improve a lot on the design there. Make sure there are no breaks, and don't make unfair segments. Use your imagination to make interesting segments. There are always lots of cool ideas to be found. I also suggest remaking these levels, as they aren't good right now.