I think that your timbers are certainly bigger than they need to be. Especially in W.Oak I would consider going down in size a bit. Without seeing the design, I would think that 7x7 and 7x9 should work. I would say you are getting a great price on W. Oak. I wish I could get it for that price!
Ideally you want the center as close to centered as possible. But as long as you don't have really crooked trees( ie centers out the sides) you'll be fine. I doubt this stuff will move as much as the small dimension R Oak you used last time. Still, the sooner you raise after milling the better.
Just out of curiosity, why assemble the bents away from the site? It sounds like it would complicate transport and loading/ unloading.
It sounds like a fun project!
