To close the thread, the job is done.

It turns out my tumbler was in a couple of pieces. The top ring, the one with the holes that align with the release channels in the main tumbler housing, was no longer attached to the tumbler. It was held in place by the black cylinder that covers the tumbler. I suspect it was this loose piece causing the key to stick, not worn key fingers.

I made the u-tool that finally worked out of a wire hanger. The diameter of the rod was an interference fit in the channels, but that's what it took. 2mm wire didn't do it. A reshaped paper clip wasn't even close. It also took a 5-degree CCW turn of the tumbler to get it to release while extracting. The manual says nothing about that.

Thanks to euro 287 and Paul for sticking through this with me.

Sixto
91 300SE
87 300SDL
81 300SD