Right, but probably 2x rafters.no vertical supports in the middle..
I don't think using 1x6's for 12' rafters is a good idea.
Also buying 12' boards might get expensive and be hard to find.
There's no quick simple answers to your questions, will take some research and planning.
I don't know enough about framing to plan it out for you, sorry.
Have you heard from the 'authorities' yet?