Dear Blake,
Egad. Your description of your problem with the reverse shift and your tale of blind alleys and deadends with trying to solve it sounds *exactly* like my problem with reverse that I endured and chased for a few years.
In short, the shift into reverse would work fine when cold. However, after about an eight mile drive, it would grind going into reverse and continue until it had cooled completely overnight.
I adjusted it the shift linkage. I fiddled with the slave cylinder free play. I changed engines. I changed clutches. I asked Dennis Quella. Finally, a local Pantera friend ( with extensive ZF experience) confidently told me it was a problem with the shift linkage adjustment. I found this hard to believe because when cold, everything including reverse worked fine. When warm, everything worked fine except for reverse.
At this point, I had nothing to lose by returning to adjusting the shift linkage. As I looked at it critically, it did seem to be ever so slight off center. When I adjusted it to the center sweet spot, the cold shift into reverse seemed a bit easier and after a test drive for the warm shift---it worked fine and has continued to work fine.
I would advise checking all your shift linkage U-joints, particularly the one under the ash tray in the center console. Once assured that they're all tight, then proceed to another ( yes, I know it is tedious and seems pointless) shift linkage adjustment. You might even consider warming it up and then adjusting according to protocol.
Warmest regards, Chuck Engles