Oof, not a fan of invisible blocks being a requirement. I've hated that ever since Lost Levels 5-1, where for years I assumed the only way past was to take the vine to the World 6 Warp Zone. It wasn't until I stumbled across a YouTube video about a "world 9" that I'd never heard of that required beating the game without using warp zones to access and I was like "wait, how do I get past world 5 without the warp zone?" and looked it up that I discovered the truth, and to be honest, I'm still not over it; no wonder Nintendo of America reskinned Doki Doki Panic instead of localizing that title.

I didn't even know that invisible blocks were in this game until now, so I guess my recommendation would be...don't have them at all? Do something else instead? Maybe in the sequel, you can design the early levels in such a way to guide players into hitting them naturally and safely and/or having obvious clues like holes or markings on background objects to indicate their location, then slowly work your way up to having more subtle clues before finally having end-game level design like what you did here..but right now, as-is? No, you didn't earn that invisible block placement. 

By the way, I realized I misunderstood what you were saying about the boss powerup not overwriting the player's current powerup because I went into the first boss with the float power, which *did* get replaced even though I would have considered that to be a powerup as well (just not one with a weapon). It wasn't until I reached the Metal Underpass boss with the Reinforcement's bomb power and died that I realized it actually gives me the Fireball power instead. In that case, I think it might actually be a better idea to have the boss room powerup overwrite the player's current power anyway so, for example, players don't end up fighting the Sand Queen with melee weapons when the game would have given them the Mega Buster instead (I went in with the cat-claw power but happened to have a Mega Buster in reserve, so I just barely avoided the issue). The thing you brought up about letting players choose their own power still works since their current one would just go into reserve where they can simply bring it right back out.

Also, is it just me, or is the Gaping Abyss a bit of a difficulty spike for world 3 due to the instant death pit? I'm sure the boss could work as-is in a later world, after the difficulty has ramped up to match it...but the easier solution might be to cover the hole with spikes so the player only loses some HP for getting knocked off instead of dying outright.

EDIT: Okay, so apparently Caldera Keep was supposed to be world 3, but I think my point still stands. Also, in Yal's convo for Caldera Keep, he says "moment of surprise" instead of "element of surprise." Was that an attempt at characterization that I didn't get, or did you just forget the expression? Will he be corrected if I pick the right character the same way Sugar quips at Tomato in her Metal Underpass convo?

EDIT 2: Actually, after playing it a bit more, I think Caldera Keep is actually harder than Metal Underpass and they should have their painting requirements swapped to represent this. It feels like this world has more blind jumps/drops than average, where you can't see if there's a hazard or floor below you until you jump down. In particular, Dining Hall of Hell doesn't have certain platforms visible from a safe distance even if you hold down to pan the camera down...unless you bring in the cat claw power from another level and climb down certain walls. Maybe you were trying to indicate them with the chandelier chains, but just like with the invisible blocks, you need to build up to that, for example by having a preceding chandelier level that isn't so dark so the chains can be seen clearly. Still way better than Fun In The Basement, though.

EDIT 3: And while I'm at it, the camera takes too long to pan down after the player starts holding down. It kills the pace, especially with how frequently the player is expected to do it. It should only take around half a second.