I have a question... Are ANY of the bosses 'actually' beatable, with each of their attacks doing well over half your life, without being 'forced' into the Bimbo levels ?
Yes, use the iron armor and poison. Once you've got a boss poisoned don't attack just keep spamming healing potions. Eventually they will take less poison damage then normal(like 124 instead 250). At that point you can one shot them so just attack. I beat all of the bosses except for fire and final one without using any bimbo spells, however you can use one bimbo spell without your bimbo level going up so I'd say save it for the fire boss(I used the electric one). For the final boss you won't gain any bimbo levels so use feel free to go wild. For the final boss I mostly used Bimbo's Best Friend(twice, you can get it by solving the cracked ice puzzle I think) to try and keep the boss weak enough so that I can use healing potions to keep myself alive and occasionally sneak in a few status effects from the other bimbo spells before the de-buffs wear off.
I don't remember that much about the class change; all I know was that you could definitely gain some bimbo levels without triggering it. As for the third scene it comes from one of low bimbo level endings, I think it's the 1st ending after the 1/no bimbo levels ending, although it could definitely come after that.