Oh, and on the notion of trolls and fire. I'm one of the ones who has no problem with newbie characters using fire on trolls. There are tons of perfectly plausible reasons why a character who has never seen a troll would know to use fire: fairly common knowledge in a D&D world being one.
But knowing to commit suicide to break polymorph? Yeah, not going to buy that one. It's not exactly something that's going to come up all that often. That someone had heard a story that you could break polymorph by killing yourself, and then actually believing that story is a very, very far stretch.
Look, I get it. I'm totally behind of the idea of not policing the players. But, by the same token, there are limits. For the same reason that I don't try to create gunpowder in your game, or use Major Creation to create white phosphorous or a nuclear bomb, everyone at the table is somewhat responsible for trying to keep things in the realm of plausible.