I think the lesson learnt here is that if you take your Tesla into service for any reason, uninstall all your aftermarket stuff first so you don't get blamed!
No, I think that's too far reaching.
Buying a car is all about maintaining leverage.
You inspect the car first, and you reject the car if there is a something significantly wrong. I myself failed this one on my P3D because it had a misaligned bumper. I opted to go the due bill route, and Tesla fixed it. But, they didn't quite fix it as precise as I would have liked. Maybe I made the right decision as I haven't had any other issues with it, and its been well over a year later.
You test drive a new car first. But, Tesla doesn't let us do this aside from the 7 day return policy. The OP modified his car so quickly that he lost leverage. There are two stories within the OP's story. The first story is unacceptable customer service on the part of Tesla. The second story is how much he got bullied. He even makes fun of himself with the title.
As to modifying a Tesla there isn't a whole lot you can mod. Sure you can do cosmetic stuff, and that's fine. You can also do sound upgrades. With the cosmetic stuff just make sure you don't mess with the sensors. With the Sound stuff just make sure everything is electrically sound. If the mod is really extensive it's probably a good idea to have the capability to disconnect it when troubleshooting some glitch. To be perfectly honest the SW is so glitchy before mods that I wouldn't take the chance on touching the electronics at all. Thankfully the P3D I have doesn't need anything except a face lift. It always looks like it's smiling, and I'd like something a bit more aggressive.
As to actually owning a Tesla? Just forget about that. Sure there are some right to repair champions working on making a Tesla friendlier to actually owning, but they have a tough fight. The serious mod people sign themselves up for this fight since it will be a fight.
You simply can't do any extensive mods of a Tesla because you can't touch the computer, and it's a computer on wheels. Even the people who have hacked their Tesla can't do much with their power.
When you buy a Tesla you're buying an experience, and not a car. You'll never really own the car. Tesla manages all the cars in a fleet management style way. If there is something that will reduce THEIR liability they can simply tweak the SW so that we lose range or we lose charging speed.