Thanks for that, Dave. Although your answer covers both bases, I should have mentioned my car is an N/A.

The boots looked good, but peeling back revealed rust on the one protecting the seized pin. My efforts to extract it included trying to twist with vice grips, whacking with a 16oz ball peen, heating bracket surrounding pin with MAP gas plumber's torch, then whacking. At this point I gave up, because I reasoned that even if I did get the pin out, both pieces would be damaged from rust to the point I'd need to replace them both anyhow.

Besides, I can still drive the car as-is--the pin must have seized a long time ago, probably prior to my ownership. If I break something and need to order parts, then the car can't be moved away from the front of the one car garage in our one car driveway. Not the end of the world, but something I prefer to avoid. There is noticeable drag on the affected rotor, but it's still able to spin by hand from the wheel studs. Interestingly, the brakes seem to operate fine. There is no pulling with brakes applied or not. This is probably the biggest reason I should just repair with correct parts, since even in a degraded state, everything seemed normal. Since I've never had a pin stuck beyond the point where I could twist and pull it out with pliers, I'm assuming these won't be serviceable once apart. Obviously pin isn't an issue--easy enough to source, as you mention.

The thing that got me thinking about the 850 calipers was seeing a kit for this swap from FCP some months ago: https://www.fcpeuro.com/products/volvo-brake-upgrade-kit-centric-8111102kt
It seems the only 850 specific parts are the calipers, brackets, and pads. When then searching to find out more, I saw a few threads on turbobricks covering upgrading to 300mm, 320mm, and beyond. I stopped myself before going too far down that rabbit hole, but it's interesting nonetheless. Your notion about the dimensions of the newer designed car's rotors differing seems to be correct. In addition to the larger rotors, and calipers/brackets from the same application, one needs longer mounting bolts and bushings, to move the calipers outboard a bit (like 1/4" or so, if I recall). Then wheel fit can become an issue--the fun goes on.