Just a small question, do both (all) players need all collectibles or can just 1 person get all the collectibles, while the rest is going for quests?

I don't know how the game tracks collectibles but if it's tracked separately for each person you probably should collect it on both players but if there's still no rules and no one has done a run of it, then I say do what you feel like and try and ask one of the mods when they stream. Nord does so every so often.