I wouldn't worry too much about breaking the spot weld nut when trying to take the bolt out . I will say shame on Tesla for only spot welding the nut in one location - it would literally take another couple seconds during manufacturing to make another spot weld. Also in terms of penetrant - Kroil is king (will use PB Blaster in a pinch). I am a hobby machinist with several friends who work in the tool and die industry and we all swear by the stuff.
FWIW I broke one of the nuts on my car trying to install the Baesnor mud flaps and replaced it with a clip nut from the auto parts store, which was a super quick and easy fix:
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I didn't like how rigid the Baesnor flaps were, especially when they would occasionally hit the ground going over speed bumps and steep driveways so I switched to Rally Armor, which are exponentially more expensive, but I think the product looks and performs much better.