Xelnath got fired. Who ever is filling his shoes had to rework the entire quest chain etc as to not encroach on his intellectual propeties and in turn have blizzard get sued.
Not really. If it was stated in his contract with blizzard that any work he does for them within their studio becomes property of blizzard, then no. The new guy can come and resume work without the need of using Xelnaths permission.
If it's anything like a real studio, the rule of Anything Created and Done within it becomes it's property.