Unless I were to offset the tenons and lap them on either side of each other in order to run the tenon past the centre(Canadian spelling) of the 10" post. How common is this?
I'm not totally sure how common it is but when I questioned Jack Sobon about how to get the relish needed on tenon he suggested an over and under tenon set up:
and to see through the joint:
Should one consider a spline joint instead?
Each frame and each joint depends on a lot of factors. Each has to be evaluated based on size, location, load, type of wood are a few things on the list. So a general answer would be maybe.....
Is this tiresome answering similar questions from different people over the years?
No, over the years we have all learned more and more. Timber framing is a life long learning process, experiences add to your knowledge base.
Am I cheating by asking questions here instead of taking a design course?
A design course will help you understand more than you can learn by reading posts on forums. But keep asking questions.
Should I just trust the Eng.?
Yes, but you may want to help educate him in timber framing basics if he doesn't have a lot of experiences with timber framing.
Should I just stick to nails and 2x4s? (please no!!)
Every type of construction is something to learn......