I highly doubt other brands would help as means of skirting the system. Polestar and Volvo are still under the Volvo/Geely arm. Plug In Electric Vehicle Credit IRC 30 and IRC 30D | Internal Revenue Service talks about manufacturers.
It sure didn't help GM with Chevy + Cadillac with their EVs and PHEVs. They hit the limit and phaseout to $0 awhile ago. They can't just start selling a whole bunch of GMC, Cadillac and Buick (for exmaple) EVs/PHEV to get the full $7500 Federal tax credit on those or rebrand the Chevy Bolt as one of those...
Correct, the tax credit is by manufacturer not brand. The Chevrolet Volt and Cadillac ELR were both under the same GM numbers. Same with Volvo and Polestar as it is with Maserati and Chrysler. VW, Audi and Porsche all run up to the same cutoff. Tesla can create a thousand vehicle brands but it is still out of credits unless the law rewritten. I guess technically Tesla could be purchased by someone with credits left (e.g. Ford) and then it would reactivate, but I don't see that happening.