Yes, with only a few mostly archaic exceptions. If it already ends in an s, you don't need to do anything. If it ends in an s and it's really, really ambiguous, you can write s' instead - but it's strongly recommended that you rewrite the sentence to make it less ambiguous rather than that.
No, it's completely ungrammatical.
It's not actually from "his" or similar. Old English used to have a -es suffix for the genitive singular, and the e in it was gradually replaced by an apostrophy under Norman influence in Middle English.
What you're referring to is called "his genitive" and is not used in English. You can read a little more about it here: https://en.wikipedia.org/wiki/His_genitive