Correct me if I'm wrong, but you only get quest credit for kill x# of y creature type quests by landing the killing blow. Since (presumably) it's your opinion that players should mostly group for experience rather than gather it solo, I think it would not hurt game balance to give 50% credit of a kill when done in a group for these type of quests.

I think the existing quests are pretty much fine as is; however, I've got a couple ideas in mind for variants of this kind of quest that would be meant to be completed as a group.

We'll see when I manage to find the time to implement a prototype, though.

It isn't just for landing the killing blow, far as I can tell, but neither is it evenly split. If I had to guess, it is randomly given to one of the group?

So far with quests like the ones you kill X creature for money, only those who landed the killing blow get paid for that kill. At least, that has been my experience.

I've done a lot of these, and I can think of at least one that doesn't work this way. I got quite frustrated because my groupmate kept getting the credit even though I was landing killing blows. So we'd kill 10, and he'd get credit for 8 of them, even though I'd landed killing blows on 7.

It's possible that I am mistaken, but I don't think I am.