1) Armor is location-based, and there are several factors in determining where something will hit: Meteor or Firestorm is most likely to hit you in the head, Hamstring will hit your legs, Earthquake or eruption will probably hit your feet. Regular attacks seem to be more random, but your chest and legs are more likely to be hit than your feet or arms(which is probably why those armor pieces cost more to craft). Armor provided by a shield gives you extra armor versus attacks that come from in front of you- it won't help against someone hitting you from the side or back.
2)Ensign has provided a nice
article on how damage and armor is calculated. Hopefully this will answer most of your questions.
3)Superior Vigor runes are worth a lot- probably several thousand gold, though I'm not really a good person to ask about prices. Since a Superior Vigor rune is good for any character, and they're rather rare, you'll probably be able to set your own price, so long as it's somewhat reasonable.
4) Most weapon types have set speeds- for instance, all swords will attack every 1.33 seconds if I remember correctly. The exception is bows, where different types of bows have different attack speeds and ranges. A shortbow attacks faster than a long bow, but has a shorter range.
Nostrau: Your characters will stay with you- they're tied to your account, and the world is all the same- no different shards or anything.