Specifically, whose story am I playing?

I'm playing two games at the moment, a single player run through as a custom character, and a coop run-through as Wyl in the second player slot.

I've noticed that in coop I get a lot of story if I instigated chats with NPCs or scripted events even while sleeping at camps.

How will the game end in this way, will player one and player two get two endings?

In my single player playthrough, if I decided to move Gael to my first character slot and control him would I get a Gael playthrough instead of my custom character? Would that mean there's no point in starting as Gael as an origin character?

Does anyone know the difference here?