So here's the final on this saga.
I was recently able to repeat the road trip from the first post. Sadly I found the charge tapering behavior returned, even though this trip was much cooler than the first - ranging from about 50f to 80f. This time I was also armed with an OBDII scanner and the Scan my Tesla app.
I noticed several things
1. On a V3 charger, the battery would quickly heat up to 150f. The radiator fan would start running around 135f, but clearly was unable to prevent the battery temps from rising. I could not hear the coolant pumps running at this time.
2. Once the battery rises above about 145, the car reduces charge rate for thermal protection.
3. Once charging had completed, the radiator fan would continue to run for a long time - sometimes over an hour. Watching the battery temps on OBDII showed the battery temp dropped very slowly - taking over 30 minutes to drop from 150 to 130f. Clearly there was no active cooling, nor passive cooling through the radiator. It was simply the battery losing heat through he casing.
4. If I felt airflow through the radiator (removing the frunk cover), it was warmer than ambient IF I had the AC turned on, but if I turned off the AC, the air was no longer warmer than ambient. This is with the battery at 150f. So clearly coolant wasn't being moved through the battery or radiator.
So the car went in for service - sure enough the "super bottle" was bad. Total repair: $1200, on a car with 59k miles. It's out of warrantee, so that's all out of my pocket. The service rep said it's not bad for a car with 60k miles to be coming in for its first repair (!!!!), and he's seen super bottles replaced at 25k miles. What?
If I'm looking at new cooling components every 25-50k miles, needless to say this has changed my opinion of the car. This is especially concerning since the health of the battery is degraded every time the temps climb unchecked. Thermal management is what separates Tesla from the Leaf in terms of battery longevity.
My tip to any Tesla owner - if you see charge rate throttling, or notice changes in how your car sounds while or after charging (no longer hear some whining pumps, or radiator fan stays on a long time) - get your car to a service center. You will have NO INDICATION (warnings, CEL) from the car that something is wrong. Don't listen to the internet "experts" telling you this "just happens" sometimes because of the charging station was busy or the handle gets hot or the phase of the moon. I put off this service longer than I should have because of those "experts" and I lost some capacity because of it.