It seems like there's a market for a company that will buy Teslas ultra cheap, modifies them heavily, then rebadges them like Alpine does for Renault, AMG does / did for Mercedes, Abarth for Fiat, etc.

These days those are all subsidiaries of the main brand, and even before that they had a cooperative relationship with the main brand. But, I can imagine a setup where the main brand doesn't support or approve of what the modifier company does.