First of all, I saw a screenshot using the Hell tileset, but I just finished the game and didn't see it...

Second, when I started a second savefile to have more lives at a certain point, the sewer level just skipped. Anyone know why?

On the easy mode, the two most frustrating levels(The one that uses the hell tileset in the last 'episode' and the sewer level) are skipped so the kids can feel accomplished as well(In a test with some kids in my cul-de-sac, this proves to be a good idea). Otherwise, I have no idea.
The Hell tileset is used on Chelonia IV, I think it's the second level on that planet. I believe Jazz complains right off the bat about never having a mission without lava.

Guess I accidentally picked Cakewalk on the second save then ^_^;;