Which map do you mean? I am not sure what you mean here. I meant you should have a Map<ServerWorld, WorldSavedData> which holds your WorldSavedData instance. You cannot use the normal way of getting it, because that uses a non-thread-safe map.
These are basic Java concepts. Objects in Java are passed by reference.
Yes, you can have issues. Multiple threads require synchronization as long as at least one of them is writing.
He's joking. Don't use Unsafe.
Capabilities is the same. Mixins has nothing at all to do with this.