With range mode off, and plugged in, if you started cabin heating now the old theory was that the battery heater would come on. So if you tried that, and then looked at the TeslaFi data to see whether or not the battery heater came on or not, that would be helpful.
If you wanted to take it a step further, you could go for a short drive, after preheating the cabin a bit, and see if the battery heater came on during the drive. Obviously I'm not going to ask you to go for a drive, though, just to test this. You could do the same test the next time the battery is cold, and you're going for a drive anyway.
Thanks for being willing to help!