I mentioned your write-up to the BMW certified body shop. They simply refused the work because there was no "BMW approved fix." They also said any attempt by them would well exceed $2200. Honestly, I think they have my best interests in mind from their perspective: a 25 year old car rolls into their shop, it needs a full body repaint and some rust repair beyond the subframe, 240k on the odometer, and there are various yet undefined fluid leaks. It does not look like a worthy candidate for restoration considering its less than collectible status.
Nevertheless, I intend to drive this car as long as is feasible, and as long as it's safe.
I intend to find an independent body shop that's a bit more adventurous. For now, since I need new subframe bushings anyway, I'll replace them with polyurethane versions. The thinking is that the stiffer bushings will not allow as much deflection and twisting of the subframe bushing pins. It will hopefully resist the advance of the deterioration at the mount points. Add a rear strut brace and maybe the whole rear end will be tight enough to drive as-is for awhile. It'll give me time to determine if I want to spend thousands on the body repair, or search for a donor with a good chassis.
I'm guessing your body man never took any pictures of the work in progress, but I'd love to see them if he did.