I don't know. Often people take Hyuuga in this because of the dojutsu, but I have beliefs that it's only a mutation from the Sharingan or something like this. Who knows. Even if Hyuuga were indeed one of the paths, there would still be 2 more needed.
Yeah, but thinking of it, it doesn't make sense at all anymore. We were loud and clear explained that Uchiha come DIRECTLY from the Sage, so there's no other clan that they could have derived from since the Uchiha ancestor wasn't related to anyone else but the Sage who made him literally with Creation of All Things.
That's less likely. He and Kaguya were the only ones having chakra or holding any meaning to shinobi clans. I don't think that the Hyuuga, even if they existed back then, could have had any of today's powers or anything alike, I think if they existed they were simply another random clan. But then again, it was never stated that Rikudou had a wife. As far as I remember, not 100% sure, I think it was stated that he created his two sons with Creation of All Things. He was a God, I don't think he could have united with a human. And, if he had natural sons he wouldn't have ever been able to choose which has which power. This still needs to be thoroughly explained by Kishimoto. We know too less about the Sage to come up with any conclusions, all we're able to assume are merely thoughts and theories, but nothing can be proven since there's less to nothing information about him.