I read "Mary Sue's" as a mistake. I by far prefer Mary Sues.

I'd use Buffys, Henrys, Louises. I think there's a rule that you're not meant to alter spelling of proper nouns.
Once the word drops out of properness into common usage (a la "to google" from Google the search engine), it's okay to drop the capital and impose grammar rules on it.

I'm not referencing anything though, so feel free to tell me I'm talking out of my arse!

(I'm Miriam from your 4002 class, by the way... so you don't screen me for eternity!)