If you're still checking this thread, I have a couple more things to add:
1) The invisible block placement for Ominous Clock Tower's third painting is marginally better since it's first one is right next to a wall, but I still only found it by chance, so that level also isn't a great way to introduce them even if Caldera Keep was meant to be before Metal Underpass.
2) If you're willing to make more drastic changes, I'd like it to be possible to kill frozen enemies in ways besides jumping at them from below. The first level after Caldera Keep's Reinforcements has lots of narrow halls, so when I tried out my brand new ice power, I ended up having to sit there and wait for the enemies to thaw before I could progress.
EDIT: Also, the Icon of Sin has a similar issue as Patient Zero did: it should have some indication of how far its melee flame attack will go, especially since the boss room gives the player the short-range sword and the Reinforcements give the player the longer-but-still-melee green whip. Since you don't want to do ! zones, maybe you could show its flame being drawn out during its wind-up pose so players can actively see its length increasing and realize how far they'll need to run away to avoid getting hit.
EDIT 2: The collision on the floaties in Water Gun Warzone is spotty. Sometimes I'd land directly on a beach ball and just clip off of it suddenly, and one time, I did a running jump off of the platform by the third painting and fell straight through one of the longer rafts, into the water.