You are not making any sense here.
There is no relation between the siblings being twins and the seal. Only one son/daughter would be heiress and another still will just be there to serve the main family as a branch family.
Yeah but you forget that branch family was to serve so the main so it was Hizashi's duty too. The fact that they were still brothers and twins are supposed to be extra close, laid a heavy burden on Hiashi who didn't want him to go though it because of the supposed "duty". Hizashi told Hiashi - he wasn't doing it because of his"duty" but choosing to do it despite it.
Hokage part is completely irrelevant. Let me put it this way:
She was part of Hyuga clan long enough to get the seal. She still didn't. More so- Do you think she would have gotten one if he married another Hyuga?
Well she married one and left them without a new branch family if we are not to consider Hianata their family member anymore.
And I don't remember Naruto declaring he will find how to reverse the seal- he said change the Hyuuga ways. As I said unless you can show us something that the system continued lease give the link.
Oh please- when you are doing something to avoid the feud and rebellion and personal risk to the main family, you want to make sure that you have the upper hand and can act fast on it even in emergency. What's point of the seal which needs an elaborate system to activate and then uses a 'fair' system as if it's tournament? You can as well lie down and die.
And Neji did almost kill Hianata. But for Kabuto, whose medic nin instinct took over and he went out of his way to heal her while pretending to be an ANBU, she might have still died. He made peace with his role and decided to still live to the fullest and in best way he can.
It may not go well with your world view and satisfaction but that doesn't make his choices any less.