New bearings arrived from the parts order.
Five of 12 bearings in the three tower skates/carriages had noticeable play. I replaced all 12, as they are not terribly expensive, $0.75 ea. The new height map using the stock "_First Probe" macro which runs both G32 and G29 on a heated bed, resulted in the following heightmap.csv
-120.00,120.10,-120.00,120.10,125.00,20.00,20.00,13,13
0, 0, 0, 0, 0, -0.113, -0.076, -0.008, 0, 0, 0, 0, 0
0, 0, 0, -0.026, -0.036, -0.061, -0.013, 0.009, 0.012, 0.097, 0, 0, 0
0, 0, 0.031, -0.017, -0.072, -0.041, -0.013, 0.015, 0.027, 0.060, 0.123, 0, 0
0, 0.052, 0.029, 0.008, -0.024, -0.039, -0.037, -0.034, -0.039, -0.037, -0.001, 0.094, 0
0, 0.028, -0.040, -0.059, -0.077, -0.067, -0.078, -0.043, -0.029, -0.022, -0.004, 0.028, 0
0.063, 0.028, 0.001, -0.016, -0.044, -0.075, -0.101, -0.096, -0.099, -0.105, -0.103, -0.101, -0.000
-0.016, -0.072, -0.095, -0.083, -0.088, -0.100, -0.111, -0.099, -0.085, -0.102, -0.090, -0.102, -0.088
-0.028, -0.021, -0.022, -0.045, -0.037, -0.079, -0.102, -0.140, -0.138, -0.130, -0.158, -0.185, -0.172
0, -0.104, -0.072, -0.101, -0.078, -0.087, -0.075, -0.097, -0.121, -0.133, -0.137, -0.164, 0
0, -0.089, -0.071, -0.036, -0.036, -0.067, -0.103, -0.116, -0.145, -0.158, -0.182, -0.197, 0
0, 0, -0.071, -0.072, -0.075, -0.046, -0.058, -0.090, -0.088, -0.138, -0.159, 0, 0
0, 0, 0, -0.034, -0.011, -0.010, -0.048, -0.073, -0.075, -0.096, 0, 0, 0
0, 0, 0, 0, 0, 0.011, 0.058, 0.061, 0, 0, 0, 0, 0
The third candidate, belts, were deemed adequately tensioned, and this heightmap suggests that to be a proper conclusion.
This looks to be just a wear-out scenario. The machine had 2500 hours of usage.