It might be an American thing, because I seem to be getting exercised about my gawd-given right to keep it simple and not go around adding apostrophes to every "s" I see, or putting an extraneous "u"in "color" and "labor". Actually, I find it quaint and charming to see the "u", but I like to keep my apostrophes strictly for possessives and contractions. Oh, do I love a good contraction!

And yes, the possessive of Joss is Joss', the possessive of Louis is Louis', and the plural is Josses and Louises. It doesn't matter if it's spelled the same as something else. We have lots of things in American-English that are spelled the same and pronounced differently. It's all part of our rugged appeal.

I would say my own personal rule is stay consistent, and keep it simple. Sometimes I fail. :(