regardless of the hatred issue, having 1 unit of B TO a region of C with the help of an allied army of A... is that a good thing, if it's indeed possible?
It's not possible. What is possible (as Anaris described) would be for Realm B to take a region from Realm A and then give it away to Realm C. Why anyone would do that—since it would take weeks of time and effort—is another question. There really isn't a reason to it since Hatred doesn't do what many people thought it did. If you want to keep people from taking over your regions, that's why we have a Diplomat subclass.
