Side View Mirror Stuff FWIW

Lets call the little end pieces crimped onto the cable end ferrules. Those 3 ferrules at the mirror end of the cables have or had a half ball shape ( approx' 1/16 radius ) facing the backing plate. The earlier the build date of the mirror, the larger that ball radius. Why they reduced the size of that ball end ferrule ?, I have no clue. I know the result is the plate distorting. Weight of glass, spring pressure, repeated jarring from door movement, combines to generate the distortion. Smaller ball=smaller surface area= more rapid failure. If you look at your 4th pic from top close enough, the light shadowing reveals this distortion. This is where the cable slack originates. Same thing will happen to earlier builds with the larger ferrule, just takes a bit longer.
I don't know for sure but I expect your patch washer will force the mirror to return to a certain position ( dictated by the 3 cable lengths, patch washer shape ) repeatedly. Good luck with it.
From my first go at this way back in early 80s... 3 small "split shot" weights from a fishing tackle box was a good fix for several years. I'm sure those aren't as good as they once were either.