I was wondering if you've been able to solve the problems with quests and quest givers where a large amount of players want to get a quest from an NPC or go to an area for the quest, how do you stop griefers from causing issues?

There's no one single solution, and when you have an MMO, there are some things you have to sacrifice. They have mission boards instead of NPCs in larger places to filter all the individual players, but in remote areas there may be a single person you have to talk to and if someone wants to talk to them as well, they have to wait their turn. To prevent griefing, it may be a godfather scenario where if you annoy an NPC, they won't talk to you anymore or have their men "Take" you away. NPC's can pickup on situations such as someone talking to them too much or etc. There will be other ways of course, such as if someone is important, you have to book an appointment, but in the end with having the world setup the way it is, there will be a lot of places to go to that disperses the players. In the end, they still have to put it out there and see what needs to be tweaked.