For the time being, at least, I will be controlling all of the NPCs in the tavern directly... in other words I will make sure that they all behave as though they were real characters and react to what Kaedan does and says the way they would have. Would there be a side-quest if one was to address the true deigan? It's possible, though it will depend on the exact progression of events. Technically, [I]any[/I] character in The Prophecy is a potential source of side-quests, if one wants to use that term about it, though it is a bit gamey.
In any event, just have Kaedan enter and do whatever he would naturally do; I will handle the rest and try to keep it interesting.