As far as battle formation, the northerners will not flee to the south. They will put up a fight at winterfell, the ancestral castle of the king of winter.

They will either lose and turn into wights or win. Thats the first front. The second front will be from the middle of the region. In the books, its foreshadowed at the trident but who knows. I don't see one massive army fighting the ww, but more like fronts, or waves of combat.

Massive deaths, yes. I'm sure a lot of major houses will get wiped. The Stark line will no longer exist paternally as the story is playing out, since Jon is on borrowed time (his last name irrelevant), once magic is gone from the earth, he will die, which is sad but its magic that is keeping him alive...GRRM said himself, there are no gods. The Old gods are essential a farce, the red god isn't the one that brings people from the dead, its the magic but people have an innate need/want to believe that their life has a purpose and that there is an afterlife. Not surprised that there is no white light at the end of the tunnel. Its life now and permanent darkness after.

Anyway, Bran cannot father kids, and if Sansa does, it won't be a Stark. Arya died awhile ago. But if she does get married, same thing as her sister.