TIMBEAL;
"simple structure like this you can fudge it" -- I was hoping for such an answer, because I am sure there will be compromises.

your comment on the rafters, are you saying that I should do a different approach?

Roger_Nair;
The ERC is forest grown with nice straight trunks with no large branches.
You are scaring me on the dimensions you recommend (I have tried to get mostly heartwood in the posts/beams). The diameter of the trees do not support 7x7s (mucho wayno). 6x8 with a good bit of non-heartwood.
I can, and will, decrease the rafter spacing.
Is there some type of stress test i could perform on my 6x6 12' beams? Perhaps hanging a weight from the mid-point of the beam?

You can see that I am grasping here to keep the original design. Do to limited supplies and what has already been cut.

Would anybody else like to weigh-in on the 6x6 12' span? Do not want to start any arguments. Maybe somebody has done a 12' span with 6x6s.