No, the Hermetic Arts aren't favored abilities, they're opened through a special ritual the first season of apprenticeship. You could certainly have a character with four or more Supernatural Abilities before the ritual, but it makes the ritual harder, and it means your master has to have a high Intellego Vim total or it could fail completely.
One tradition in your lifetime. It's not always easy to determine when someone's joined a tradition, but basically when you learn something as a favored ability, you've joined the associated tradition. If the source you're learning it from doesn't have a tradition, you can't take it as a favored ability.
There are different traditions of Hermetic Magic, too. Mercurian, Faerie, Diedne, even things like Mutantes magic. Many of them have been converted into Hermetic Virtues, though, and once they become Hermetic they are rarely compatible with the previous tradition any more (Holy Magic and Chthonic Magic are two interesting exceptions).
Alas, no. He'd probably be stuck as a hedge wizard.
Any supernatural virtues that you start with, you have to take as Virtues. Ex Miscellanea get one for free, as you note. If a Damhan-Duidas takes Giant Blood as his free one, he can learn any of the other four during play, by seeking out others from his tradition and learning from them.
Yup. There's even a couple of examples of this in the book, like Aqrab the Conjurer (pages 58-59).