And now we've come full circle to the post I made earlier. Why must we get the XP for solving the quest? Since the XP would more naturally come from doing the actions needed to fulfill the quest objective. As an adventurer, especially with proper motivations, you are going to accept and complete the quest anyway. By attaching an XP reward, you are just incentivizing taking quests, which may even be out of character.
The way I see it, XP will be given somewhere. Either you are given XP for performing actions, like, say, killing dudes, picking locks, etc., or you're given XP for doing the quest, whatever it may be, or some combination thereof. If the XP is purely for killing dudes, then this basically invalidates any non-killing-dudes solution. If the XP is split between actions and quest reward, then you're incentivized to solve the quest in the most inefficiently action-intensive and perverse manner possible: Pick every lock, evade or persuade every dude, then kill them all anyway. The only equitable solution that doesn't favor a dominant path, often one that is ritually perverse, is to reward mission completion, and nothing else.
Your drawback to this is, obviously, as you mentioned, that you are incentivizing taking quests. But, as you pointed out, as an adventurer, you're going to accept and complete quests anyway, so the incentive here has minimal effect as far as distorting player behavior goes.