So, I was able to get all the parts to fix the driver's door detent, and managed to cross-thread the T-40 torx bolt that bolts the detent to the car body. I imagine a thread restorer can fix the problem, but don't have any idea which kind would do the trick. Anyone care to comment?

You are right- It's not the weld problem. The roller is on a spring that can break off inside the door- a different problem. The part can be replaced fairly easily- I've done one on our '96 850. It's been a while but I think I had to remove the door panel. Then the roller assembly unbolts. Check the Haynes manual if you have one. It was a bit tricky getting the new part in the hole- do not try to move the roller on the new part- it is spring loaded and will come apart.
daveh

If you're up to it, here's a fix posted by someone else:

http://au.geocities.com/ozbrick850/door-stay_fix.html

I had the exact same problem and saw that my hinge was split inside the door and the welding came loose. Took it to the dealership because it was covered under warranty at the time. The fix they did was exactly what was above + welding.

It cost $50 because it was covered under the VIP warranty and I had other things done under warranty too. Amazing, the parts to fix were just $8. The labor would have been another $100.

Doesn't hurt to at least get an estimate (they should do plenty of them). Or have an hour or so ready on a Saturday.

same thing happened to my 850,front and rear driver side.i removed the whole roller assembly and keep hold of the door.park uphill.