I'm not really for the 2p2p, we agree their. I just understand why people see it, why it's applicable in today's application. Regardless of it being fruitless.
Again if it starts with Seth's line then it becomes a single line at noahs time- the flood. All were killed except noah and his family. Though to argue that both "people"; Gods chosen and those opposed to him were saved during the flood defeats the purpose of having a flood.
Abraham being the start of 2 people is the only rational conclusion, since their hasn't been a cleansing of the entire earth since. Which is why they go to Abraham as the start and not back to Seth.
I see why you can draw that conclusion of going to Seth, just doesn't really follow through when you account for the flood though.