redthreaded for your insights and advice - I was really ready to give up on it entirely before your comment!
So I cut out yet another toile, this time going back to the original grain lines and not trying to make the center front end up on the salvage. I also perfected the deep box pleats in the back and added a smaller (1") box pleat to the side seam this time.
I tried the toile on both with and without the Chemise gown on underneath. As before I was super happy with the back, not so much with the side and front. It was much better though. I decided to not only add a bit of ease to the waistline of the side seam but also to ditch the box pleats and go with a "princess seam" there.
It doesn’t at all strike me as an 18th century solution to my fitting problem but it's the only way I can get it to work, thanks to my *ahem* natural bum roll. Looking at the robe from L’Anglaise et le Duc, it's actually the opposite of mine, princess seams in the back and a box pleat in the side seam. Interesting but I still question the period correctness.
So I think I've got this to a decent point. I've traced out a master pattern but I'm hesitating to actually make it up. I don't know I'm just not thrilled with it and I hate to use up my silk taffeta and my time on something I'm not thrilled with. So I'm letting it lie for a while and pondering whether I want to get over myself and continue on or more onto something I can feel a bit more that "eh" about.