As far as I understand, the Origin quests are separate (or perhaps layered on top of) the main quest line. However, the main quest line is basically the same regardless of your origin. Your main quest line relates, in some way, to the fact that you are a sourceror . All 4 members of the party share this trait and so share an interest in the main quest.

None of the party are mortal, as such. It's designed for 4-player co-op, and the they have stated that permadeath is not going to happen. I'm not sure if they're planning to give you a game over screen if only the player character/s die, but it seems unlikely based on current information. Theoretically, there's nothing stopping you from killing the player character, and simply choosing not to resurrect him.

Yes, one character will be more important than the others. When you create your character, you can choose which one will be most important and play him as the protagonist. I also understand that if you're playing multiplayer (which is drop-in), then the party member they control can start competing for the role of protagonist.

I'm sure there are some people who are so committed to role-playing that they'd enjoy competing against themself, but this really takes a special kind of roleplayer. I really imagine that most people who are so interested in actively creating (and not just participating in) a story would be much more interested in creating their own campaign in the mod tools (assuming it's usable).

Just because only the player characters can be protagonists, it doesn't mean he's the "chosen one". When you watch a movie about Nelson Mandela, he didn't achieve great things because he was chosen, or because he was born with a special power that would change the world forever. But they made a movie about him because he achieved great things. When you start watching the movie, you do it knowing that he's the protagonist and you'll be watching to see what he achieved and how he did it.

I really don't understand how you're conflating protagonist with chosen one. I love Game of Thrones, but this is not Game of Thrones. We don't need to have the expectation that permadeath can happen at anytime.