After being defeated by the rusty/intensely stuck bolts on both rear calipers, I bled the brakes, put the Audi back together, and had to take it to my in-law's barn for very temporary storage since my mom needs the space for a party she's having on Saturday. Looks like I'll probably have to detach the brake line on the rear calipers further up and probably replace the line itself plus the calipers. (Problem pictured in the second image)
So when the oppo+ party in Illinois this weekend was first planned, I was hoping to take the Buick. Then that was sidelined by general unreliability, but the Audi was making progress quickly, so I hoped for that. But it looks like it will probably take me another week or two to get it mostly sorted to where I'd feel comfortable driving it long distances. So the barn is home to the losers who can't make it beyond county lines. May they both escape the clutches of unreliability soon.