I understand the disappointment OP but I don't know why you assume it's a major mechanical failure. I wish I had a firmer ride myself, it's a performance car after all. Longshot but can you find somebody in your local club to look at it with you? Returning a car is a huge hassle and clearly you want a Tesla.
"Look. I'm still in my return window and I'm nervous. Either tell me what's wrong and tell me you can fix it quickly or I am considering returning it".
100%.In the interest of time, I'd suggest just showing up and seeing what they can do. Forget booking or calling - just get there. Easier to ask forgiveness than permission sometimes... Worst they can do is say no, but I doubt it. Play it cool, play it friendly but ask that they really, really take a look because you're in this major bind. Play on some heartstrings!
"Look. I'm still in my return window and I'm nervous. Either tell me what's wrong and tell me you can fix it quickly or I am considering returning it".