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Battery heater diagnosis

It's really cold now. I keep my 2014 p85d in the attached garage (around 60 degrees). Monday morning my 50 mile commute was in 7 degrees F. Car did okay since it was in the garage. It sat outside for the next 10 hours in the temps ranging from 7 to 10 degrees. When I left work the temp was back down to 8 degrees. I did preheat it for about 5 minutes before I left. When I got in the car I had no regen of course. Car ran fine, inefficiently, but fine. About 30 miles into my commute I get a warning saying the car may not restart. Seemed to run fine with the warning up for the rest of the drive home. When I get home I put it in park and then I also get the warning that acceleration and top speed is reduced. I suspected the battery heater was bad judging by some of the research I did on this site and others. The next day I contacted the SC and asked for a remote diagnosis since I live about 60 miles away and also have to work for the rest of the week. The remote diagnosis was for electric battery heater replacment at $708.50. My car is out of warranty so it will be an out of pocket expense.
Later that night I decide to look into it with Scan My Tesla to see if I can tell what's going on. I see in the SMT app that the battery heater is at 0% and the battery coolant pumps are at 25%. I remove the frunk cover and turn the car on defrost in the app. I can hear the battery pumps running faster and can feel coolant running through them when I touch them. I see in the app that the pumps are now running at 100% and the battery heater is also running at 100%. I let it run for a 5-7 minutes and I can feel that the lines running from the battery pumps are a bit warmer than other coolant lines that I can see and feel. The battery heater is obviously working at some level. Is it possible that it still works but it just isn't working very well? Could it stop working and then start working again? I don't mind replacing it if it will fix the problem -- but I certainly don't want to replace the battery heater and then have the same problems again. Does anyone have any knowledge on this subject that they would like to share?
I had something similar on both my 2016 Model X 90D and then a few months later on my 2017 Model S P100D.

For the Model X, here is what they found and replaced under ESA.
Model X.png

For the Model S, here is what they found and replaced under original factory warranty.
Model S.png
Same here. Replaced under warranty, but is certainly an intermittent failure. Outside of the warning msgs, the car drove completely normal. I’m sure it would’ve gotten worse over time, which could expose you to being stranded so it’s just a pay now or pay later scenario.
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Same deal last month as OP, got the error the day after we drove north into 30-40 degree weather looking for snow and SuperCharged. OTA log review and then in-person review in service center confirmed the electric battery heater as the culprit. Don't have the bill in front of me but $700ish sounds right.
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