My experience has been that brining fresh (not cryo-vac packed) birds can yield very noticeable results, while those already in a solution does not provide much impact to the finished product regarding retained moisture or added flavor. It's an osmosis thing, and when a bird has already been soaking in a salt solution, you have to use a solution with higher concentration than already used, or yours wont penetrate the meat. If you do raise the salt concentration too high, it can result in very salty birds, even if you used a no-salt dry rub.
Pops has done alot more brining than I have...I'll send him a PM and he'll be here shortly, I expect.
As for your other questions, I smoke the entire cooking time as long as my smoke is not excessively heavy.
Apple is a very good choice for birds...oak may be too heavy for the lighter flavors of the chicken, but you could use a blend with a smaller ratio of oak, say 75% apple to 25% oak. Pecan, if you have any, is nice with hickory at about 2/3 to 1/3 ratio.
Cook to minimum internal temp of 165* as measure by a thermometer in the breast and thigh, but I prefer just under 170* in the breast and just over 170* in the thighs, for less pink near the bone...165* may show some pink, so use your own judgement there...we don't like pink bird meat here.