Ok, played everything again.
I see that you already fixed the problem with the tanooki powerup at the start, and the one with the wraparound (extending blocks outside to screen so you don't fall)
So, to start with: You have a level with one main gimmick, which is the mario block.
Now, you should design your whole level around that gimmick.
Introduce the gimmick, let the player learn and master the gimmick as he plays, and ramp up the challenge the further he progresses through the level, test the player's mastery of the gimmick throughout the latter parts of the level.
What you did:
Section 21:
You have a message box explaining what a Mario block does. Wasn't the mushroom supposed to do that already?
"Show, don't tell". You can teach the player how the gimmick works without explicitely telling. Show the player how different objects interact with the block, and let the player experiment.
BTW, I found your secret exit here by accident. Being "poisoned" is not much of a reason to have an invisible wall there stopping you from entering a castle. Mario looks pretty healthy, he jumps around and everything. Why is there an invisble wall?
This whole exit is "do that nonsensical thing" that would be labeled as an "absurd second star" in MAGLX2. You actually give the player a hint once they've already beaten the level, in most cases two times. But even with the hint, it's still a nonsensical thing. The hint isn't really a hint either, it tells you EXACTLY what to do, so the second star doesn't add any challenge to the level. It's just a "thing you HAVE to do". I don't think a player should be rewarded for consuming a poison mushroom. There's not much of an excuse, or story relevance to have that exit exist.
Section 20:
Then, you have a tutorial that the player is very likely to miss.
You enter the castle through door A, but trying to leave the castle through door A leads to somewhere completely different. That doesn't make much sense.
So, this room has mario blocks, but you cannot pass through them!? The player is now confused. What happened? There's another invisible wall blocking your path! You kinda ruin your own gimmick here by making mario blocks the player cannot pass through. What's the point of teaching a player how a mechanic works, when there's no guarantee that it'll actually work? You should remove those inconsistencies from the level.
Now, after reading a sign AND pressing a switch (why do I have to do two things?), we see the dry bones, and the poison mushroom, and how they interact with the blocks. You don't need the dry bones to tell the player how this thing works, the player has eyes.
You also kinda teach the player that falling into bottomless pits makes you teleport to the top, even though that mechanic is not used in the level, except above a lava pit later.
All in all, there is lots of room for improvement here. You can teach the player about the gimmick without having him reading all the text.
Suggestion: SCRAP the whole tutorial, and expand the first part of the castle, or even the section before the castle, and teach your player the gimmick there through "play". Let them learn as they play. Make easy challenges they can't possibly fail, and let them learn.
Section 1:
The actual level starts here. Note that the player most likely skipped the tutorial and hasn't fully understood the gimmick yet.
The player is instantly presented with a tough jump above a death trap. In the earlier version, the Mario blocks weren't even visible, and I jumped to the tanooki suit and died.
In this version: there's just too much going on for the first challenge in a level. You have a long jump, 2 "interceptors" (the Thwomps), a mushroom generator and a roto disc. Jumping from thwomp to thwomp is actually a difficult thing to do. I just made a long jump below them.
Since the player is not familiar with the gimmick yet, he/she might also expect the Thwomps to die in the lava.
This is followed by 2 more small platforms with double roto discs and an unreasonably difficult to get red coin.
Why are there actually red coins in the level? Does collecting them even do anything?
Somehow, after this whole ordeal, and another spawner/bowser statue that has nothing to do with the gimmick, the level becomes significantly easier. Usually, your level is supposed to become harder the further you progress, not the other way around.
After the first boom boom, there is a dragon coin in the ceiling.
Since this is a Mario level, and we cannot see the ceiling above the dragon coin, I stacked up enough blocks to see if I could go "above" the level, like in Mario Bros. 1-2. Well, you can't.
I suggest you replace the whole coin up there with a pipe that leads to a bonus room. Inside that room, you'll have to make use of the Mario block gimmick and if you succeed, you are rewarded with a dragon coin.
Next is the lava with 2 podoboos. The first time I came across this screen, I podoboo jumped over it. I didn't even realize that you're supposed to use the mushroom blocks, since those were obviously placed there to get up to the dragon coin. BTW, the red coins are placed so far down, it's far too difficult to get to them. Podoboo jumping by itself is already quite difficult to do in SMBX.
You should somehow incentivize the player to throw a mushroom block onto the mario blocks. Place an annoying and difficult to avoid enemy there, so the player naturally wants to get rid of it with a throwable item. This will give the player this "aha!" moment when they see that the mushroom block doesn't fall through the Mario blocks. I actually think that alone is enough of a challenge this early in the level, meaning the podoboos aren't even needed. (I'd probably also give the player a set amount of mushroom blocks, so it isn't too easy to build a whole bridge)
Now we get to the second Boom Boom fight. The leaf is kind of a trap, I don't think you had that in the first version? I already have a fireflower, so all I do is sit up there and fire shots down at Boom Boom who cannot hurt me in any way.
Shouldn't you at least be forced to enter the arena with Boom Boom before the fight starts?
Trap/joke before the mid point: Was that really necessary?
The warping also changes Mario's X coordinate to be centered inside the warp area, which feels really janky as it moves the camera to it and kills your momentum. And why did you add lava at the bottom? If you really wanted the teleporting, do it with lua (yes, I know you want to avoid it for some weird reason), so you teleport up there smoothly. You remember the first boss (magician?) in the MAFAB relay level? Remember the vertical wraparound it had (made possible by lunalua)? It's just like that.
Section 2:
This section is..."busy". I think you used too many different enemy types in a very tight space together.
You also did what I already pointed out before. Mario blocks you cannot pass through! This is not only confusing, you can actually die bumping your head against the block and falling down into the bottomless pit.
The way to the dragon coin:
You have to jump onto the invisible blocks, which is pretty unintuitive. You can also use the 1-tile gaps to glitch through the wall to the dragon coin.
I suggest you remove the blocks and make the player use the leaf + the wraparound to cleverly fly from the center to the left, wrapping back to the right side of the screen, and then hovering to the dragon coin. You can also replace the fireflower powerup with a second leaf in case the player gets hurt.
Or...if you want to actually stick with your gimmick, which I highly advise you do, redesign this whole section.
Section 3:
This starts with a rather awkward jump onto the coin block, which is placed so high you'll have to do a running jump or a jump off the dry bones.
Then, the moles. I actually died here because: The mole train moves past the 2 spike things. Then, a hammer bro hit me and I lost my fireflower. The moie train also turned around. I had another flower in reserve, so I timed the power up so I could collect it around the spike thing. But then the moles turned around and I died mid jump. Those 2 red mario blocks at the bottom...those weren't there before! And they're really easy to miss, too. I know why you put a wall there, but the kind you chose to use didn't work too well.
The thwomps and the bomb: yeah, they're nothing special, and don't really fit your level's theme.
Section 18:
My main complaint with this is: this section seems to expand more and more! What the hell!
Section 19:
I was going to jump into the seemingly bottomless pit anyway, even before reading the sign. But if you think you have to tell the player, why don't you move the whole thing one block upwards, so you can see the bottom of the pit?
Careful, you actually have to AVOID the star in this one. It would probably feel better if the star and the (mario 1-ish) Mole staircase were at the end of the section, and keys/locks before it.
And why does your castle have a wooden door?
I highly suggest you watch the videos in this thread:
viewtopic.php?f=11&t=15953
All of them! Your level is the kind of one Mark Brown talks about in his videos. You could really improve your level if you watch these.