Were the Xi'an actually willing adversaries in the Cold War, or was it just the Messers using a conveniently scary neighbor as a means of controlling the empire?

I'd say a little of both.

Yeah. I mean, what kicked it all off was basically some human terraformers came charging into a system and just started terraforming a planet without checking to see if it was uninhabited. So that doesn't send a great message. So there was definitely some antagonism from the Xi'an.