Sure, as you say Fed would rather win the USO than beat Rafa today. But he can try and achieve both I guess... And his rivalry with Rafa is special enough that IMO he would really like to prove to himself that he can still beat him.
Yeah, that's right, he'll want this, and tbh it's a better opportunity over best of 3 to get a fast start and try and hold on. It's not an either/or scenario, it's more about who the better tennis player right now is. Rafa won't be thinking *eh if I lose this one nevermind I'd rather win at USO*, he will be expecting to beat Federer both times, and will do so.
Federer should consider it a success if he takes it to three sets... The only hopeful sign is that he did start to play a bit better the third set against Haas