That doesn't really change anything, unfortunately (and not really viable for someone looking to regain all the XP available during the investigation). All the other quests need to be picked up (many completed within town), which requires a lot of legwork through the town. A whole lot of dialogue needs to be clicked through in the process. And an insane number of containers need to be searched through while in the town to help finance your adventures.
The amount of XP is highly irrelevant, constantly trolling and provoking a certain woman in a certain village later in the game will give you more XP for your charm than you can get in whole Cyseal. (4500XP, each time you talk to her, one of the few characters in the game that is worth alive more xp than dead)
Actually, killing whole Cyseal will give you more XP than doing the quests there most likely ;-)
So the argument that it is all about XP optimisation is moot, this special case gives you most likely less XP than one mistake when sneaking through the blizzard of spoiler.
