Oldest child in the main family = the only child considered part of the main family. This is why Neji's father, who is Hinata's father's slightly younger identical twin, is not considered a main family member.
The whole marriage thing is fanon.
I'm sure if this is Naruto we're talking about, he's not going to charge at them with guns blazing. He'll just give an inspiring speech
It's true that there's no proof in the manga. The whole "Hyuga can only intermarry" is fan-made unless Kishi says otherwise.
The only thing is why does every Hyuga have black hair, white skin, and white pupils? I mean, the eyes are probably the dominant trait but still there should be some sort of variation like Shino's clan. Same with the Uchiha, black hair and black eyes. There's nothing that really states that they married outside the clan.

















