I think the exterior render is key, and yes I would have done it if there had been easier access.

I appreciate the "cold-roof" idea, but in this house, and nearly all of them I design, I was trying to utilize the full vaulted space by including a third story "hay aisle" if you will(there are some pics you can see through my blog link below). But why not a hip roof if you are going with a flat ceiling inside? You know, like a rancher?

Honestly, exposed TF roof systems over stone, cob, brick, cordwood, rammed earth, straw clay, woodchip clay or even straw bale walls (think sling braces, and crucks and steep thatch or slate roofs)- that's what I find exciting and rewarding to build.
But in my mind, some portion of the full roof has to be visible with several loft/jetty spaces nooked in, and maybe some eyebrow dormers if time allows.
And if speed or something else dictates that you insulate in a more "conventional" manner, then blow 6 - 12" of dense pack cellulose into a larsen truss wall system of light gauge material and be done with it. Clay plaster skimcoat over blueboard interior, wood siding of your choice and hung in any orientation (shingle, clapboard, board and batten, etc.) on the exterior.
Q:"Is the reason for not infilling against a permanent wall surface due to the drying ability?"
A: Yes.