If you have a Yarrow Flower when you approach the madman on the beach you can give it to him to avoid combat. This then gives you the option of turning in the quest to Magister Yarrow. If he hasn't been killed you will be rewarded with more XP when you do this. When you speak to her on the beach you will also be rewarded with knowledge of the secret entrance to the Fire Slug Cave. I'm actually not 100% sure this is more XP, than the 2 kills, but it gives XP and the option of the location, and requires 0 fighting. I'm not sure if Magister Yarrow's kill XP should be counted against the "good" option anyway, as the attack against her would be unprompted.
No, she doesn't know the entrance to the fire slugs, Amyro or how he is named, the guy in the cage knows that entrance. Yarrow will give you a key to room of the high judge, but she will keep the ring with restoration. But like all Magisters she will turn unfriendly after your successfull escape, so you will probably have to fight them anyhow except if you avoid going there again.
Another:
Just past the Blue Ambush boss there is a weakened guard that will attempt to arrest the player. If you opt not to kill him it will prompt several skeletons to spawn and attack. The XP for killing them is more than what you'd get from the weakened guard, and he lets you go free after. I don't think they spawn if you just kill him.
Yes, they do not spawn otherwise.
That XP don't feel given according to difficulty of the fight is at the moment one of the core flaws in the game in my opinion. 600 XP for a silent monk or 1000 XP for Voidling at the ruins feels far to overpaying while more difficult fights seem to yield less. But I can't recall all the numbers.
