I spent a couple more hours fiddling last night and finally found a way to mount my Tubus rack with the 203 mm disc and without any interference between the two. It took a bit of inventiveness, and a different mounting method for each side, but it is now super-sturdy and lined up nice and straight, so I'm happy. Taking another stab at it with fresh eyes was apparently the best plan. I still enjoyed my little exploration into estimating rotor heat capacity differences, I hope others people did, too.

BTW, I prefer a rack with three widely-spaced stays to give side pannier bags lots of support, so the two closely-spaced stays of that Bontrager rack don't look like they would work that well for me.