These are all excellent questions. If I'm writing the plural of a name that ends in "y" I just add an "s". Buffys, Henrys. I don't know that I've had to do that with any name ending in "s", so I'll have to think about that one. I think I'd just try to get around it by writing something else. lol (Just checked the style manual I use and it says just add "s" for Buffy, but it doesn't have a rule for proper nouns ending in "s". Not helpful! I think with the ones ending in "s" I'd be tempted to use an apostrophe.

ETA - I see there was a discussion further up about possessives and apostrophes. It is now generally considered most correct to use 's even with proper nouns that end in "s". However, for proper nouns, the Giles' house, Louis' hat is still considered correct as well. Consistency would be the rule there, although I will admit that I tend to say the word aloud to myself and I usually go with whichever version is the most easily pronounced. :)