Sorry to double-post, but I found new info about this game!
[Edit: Oh, gosh, I'm stupid. That video was REALLY old news.]
Someone on GameFAQs posted the names of the bosses in the game, right out of the strategy guide!
SPOILER WARNING!
"Bowser Jr. (Every single mini castle)
Bowser (W1)
Mummipokey (W2)
Cheepskipper (W3)
Mega Goomba (W4)
Petey Piranha (W5)
Monty Tank (W6)
Lakithunder (W7)
Skeletal Bowser (W8)
Bowser Jr. + Ressurrected Bowser (W8)"
Well, I guess that answers anyone's questions about the skeleton Bowser. I don't know if that really bothers me that much anymore. After all, at least it's not mindless, gruesome cartoon violence like Itchy and Scratchy is. And Bowser must be alive at the end of the game, anyway.