Didn't had the courage to read everything, but i think rare monsters should give XP instantly if you make them stop being a nuisance (Making them go away, stop or kill them)
I've seen example with the ogre quest, to make the player feel s/he accomplished something, getting the ogre stop whatever wrong he's doing should grant XP for the action. Once you've made the ogre stop doing whatever was wrong, coming back to kill the ogre won't grant XP as you already got XP for making it stop whatever wrong that ogre was doing (no double XP degenerecence some people talked about). Coming back to say "HEY ! I did whatever you asked me to do !" would grant XP too, as the task was finished. Since the game was already designed for quest XP only, to don't unbalance stuff, XP of this quest could be split 50/50 for doing what's required and reporting back.
Discovering a new area should give XP instantly.
Why ? Because it gives you the feeling you progressed, by discovering the area, by making that ogre stop annoying people.
XP Quest only make you feel you didn't progressed when the ogre died/gone away. There's no positive feedback for the player.
I don't see how this solution would break balance as every play styles would get the same amount of XP, only downside would be people not exploring by themselves would get not XP, but are you playing a role playing game to follow the roads ?