I would leave some room for movement that will occur within the 9x9 over time. I made that mistake once before and when the post started to check (and it didn't take much) the stucco finish cracked and I wasn't very popular with that particular home owner! In lieu of the 2x6 in the corner, you could try a drywallers structural galv. metal corner in order to eliminate the thermal bridging at the corner. Just cut the plumb line at your corner, screw the piece of corner on, and sheet up the other side.
Have you ever tried the Simpson let -in straps instead of sheathing? It's a method that we used instead of sheathing, when we used to build homes with a brick or stone veneer. It's easy to use, effective and saves you a lot of money on sheathing an sheathing labour.
I apologize for the confusion in which home this system is being used for. I build houses for myself from time to time and tend to call everything my home until I pass over the keys!
I'll see if I can get my scanner working and I'll try to post a drawing of what I was trying to describe. Unfortunately, this forum is the extent of my computer ability, but I'll try.