Another thing, replace the plastic heater t's if you haven't already.TB/WP was done with Aisin parts 2 years ago at 199K. Denso radiator installed last month. Toyota t-stat installed 2 days ago (old none-oe one was about 30K miles old).
I just started watching the coolant temperature recently, after the radiator popped out the top seal on my trip to MD. Comparing to my other MDX, I noticed the LX tended to run at about 10~15 degree higher coolant temperature under similar weather, which concerned me . I saw those close to 220 temperature during 110+ degree days, maybe that's why. Did see 220+ several times but it was under 115+ degree with AC on 4L climbing a steep slope.
I believe you are right and all these tests could only conclude a working but may not fully functioning (therefore weak) clutch.
Guess I will keep close watching of the cooling performance during the July 4th long weekend trip, and would most likely replace the clutch after that.
Sounds like a fun scientific experiment. Since the clutch has to be removed, I will probably just replace it with new one and leave the test to my daughter