The 800lbs gorilla in the room is that having an agreement with Chinese government, first of a kind, to set the factory without JV partners gives Elon, who is brilliant and shrewd negotiator, upper hand in any negotiations with a JV partner, if he sees fit to have such an arrangement.
As far as limiting IP exposure goes, there are many ways to set a JV, **if needed** to get rid of 25% tariff. One way would be to manufacture batteries and drivetrains at the Nevada GF, and then partner with non-automotive company (Tencent for example) to do car body and final assembly at the factory in China.