Perhaps I am misreading the drawings, but unless the two story posts from Heartwood are too small, I would try to eliminate the back to back posts by running girts from the 3 posts on the gable wall to the two story posts and housing/mortising them to carry the girts. If the capacity of those posts is an issue you could add a jowl to carry the girts. It could be cleverly and tastefully done.

Since the girts and summer are only spanning 12 feet, size wouldn't' be huge. The joists would only span 8'. Since there would not be a continuous tie, if you need tension capacity in the joists (and it looks like you would), connect opposing joists across the summer beam with an appropriately sized simpson strap let into the joists and summer to be flush once installed. Flooring covers it and you have a strong tie wall to wall.

Alternatively, eliminate the central post at the gable wall and the summer beam and just run 16' joists from eave to eave.

I have never liked back to back posts.

I'd love to see photos of the process and finished frame, so don't forget us and good luck!

Gabel