Another thought I just had would be to check the air gap as the disc is being engaged with say a feeler gauge looking for irregularities. While this would not expose contaminated / irregular surfaces it might expose a bent disc, warped PP, or bent input shaft
This can easily be done by adjusting the length of the clutch rod.
When I was going over the linkage looking for irregularities, I did adjust around on the rod just to see what would happen at various engagement points. It shook at all of them.
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