Good luck with Tesla, do let us know the resolution in the end. Your situation raised some questions that were of interest to me, so you got the benefit of my learning (e.g. how to size feeders for multiple motors/HVAC units). Hopefully I was still able to give you correct answers.
Just slightly confused by the phrase "lug nut", from the picture the only connection I see that you undid was the feeder neutral to that subpanel, which uses a slotted set screw in the neutral lug. Were there others?About the torquing the lug nut, thanks for pointing that out. I actually tightened it. Did not realize the electrical screws also need to be torqued appropriately.
Anyway, FYI, when tightening a slotted set screw like that, it's important to use a slotted driver that fills or very close to fills the slot. If you use one that's too small, it often can't transfer the required torque and you end up damaging the slot in the set screw instead of tightening it.