The first area seems fine now.
EDIT: Oh wait, there was one thing I found...
See that explosion up there? That's a Baron Von Zeppelin spawning over a platform and thus, posing no threat to the player. This happens a few times later in the level. Speaking of which...
It looks like I can now enter the other parts of the level, so let me go into that now. It's actually pretty well designed for the most part, but there are some issues that need to be addressed.
There seems to be a layer priority issue that occurs when Yoshi walks into the pipes of this level, but I don't know if there's anything that can be done about it for now without removing a layer of the background or something.
There are a couple instances in your level where Baron Von Zeppelins spawn too high and thus, have a tendency to harmlessly drop their bombs into the ceiling. Lowering them by a tile or two should fix this.
This one is completely harmless as a result of spawning too high in the screen below and just floats past the player.
...while this one ambushes the player with an explosive from off-screen!
This spot of the level is just horribly unfair. Not only is the player being assaulted from all sides by Blowhards and Bombing Boo Guys with very little ground to avoid falling into the thorns, but there is also a passage below here that requires the player to throw eggs at those thorns. It takes quite a few eggs to open up a safe passage, and there's no breathing room for the player to actually accomplish this feat.
I'm not sure what the best approach to reworking this segment would be, but I think adding more sand tiles here and making the Boo Guys above carry the bombs from the left side instead of from the right (where they just drop down without traveling any distance) would alleviate the unfairness.
This would be a good place to add another Midway Ring. This level is quite long and tough; it could use another one between the first one and the boss.
I can think of a lot of places where these two Red Coins would feel more rewarding than just handing them out for mandatory progress.
I'm not sure if you were aware, but the bridge generated by this cloud gets stopped when it runs into a coin. You should stretch this platform down one tile (along with the thorn the cloud lands on) to fix this.
This entire 3D room just shudders annoyingly. I'm not sure what you did when you changed the original 6-4's 3D lava room, but it's not easy on the eyes.
Also, that red bridge in the screenshot becomes wavy when placed over the lava. It... looks strange when you consider the rest of the solid objects don't do this.
As for your concerns on the difficulty, I think it's fine for World 6 aside from that one spot I mentioned. You shouldn't need to feel too guilty about adding challenge to your level so long as it's reasonable and not too demanding in terms of reaction time and precision.
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