[Sei]

I think having different motivations would be the most realistic way to tie the protagonists together. I agree that survival is not going to do the trick and I like the way you phrased it above.

By having their own agendas, beyond the necessary survival, it will eventually lead the protagonists together. Humans could want to get away and live normal lives again, or just be fleeing for the lives of them and their families. Demis could just want to live with the humans, or they could feel unable to do anything because they're less powerful. The vessels, being gods themselves, could want any of the above or could be completely dedicated to stopping the rebellion.

Eventually these motives would lead them to come together.