i can help you with some of them.
1) the answer is NO, the rate will keep the same 2-5 and i think in later maps will be harder to find a place with Certain gathering chances(100%), the 2-5 rate can be increased by erning crafting points (CP) if you reach 100CP in a profesion you unlock the next Tier and you gathering rate increses to 2-6 and i supouse it increase more if you have 200 then 300 then 400 CP in a profesion.
2) i think so... Premiun reduce every action to 1 AP (but i'm not 100% shure about mining)
4) the chances that the opponents choose you or any tipe of mercenary are random, if the mystic is chosen in the first round the opponent choose again, but is allways random. Later as a warrior you have some skills to "force" opponents to choose you allways first, this skill is "Taunt"
5) i think in the "News and announcements" theres is a post witht he skills of each class.
6) the benefits are many, echanging knoladge, crafting resourses, items.... like you will be a miner, let's say armorsmith.. some armors will require Leather (from hides) to be made, so you can ask to your guild to give them to you and you give them bars and stuff like that. All crafting proffesions in one point need others resourses.
If you have more questions just ask
And if you are looking for a guild in Beta server mail me, same nick.
Good luck!