For what you said about the AI going off and doing it's own thing. It could be programmed in phases. So if you're in phase 1 of a quest, the AI might be in some other part of the world. But if you get to phase 2 of the quest, the AI would then show up and possibly try to compete with you against an objective. On phase 3 of the quest, you may have the choice to join back up with the party member.
Just my thoughts on how it might work in SP.
