Quite frankly, The north should take Terran, Aurvandil should take Barca and the Lurias should take D'hara. Then the 3 factions will be within striking distance of one another and the real wars can start. Unless of course you like the idea of paying Terran to fight a war for you or you like the idea of a gang bang.
So, you find more interesting three huge hegemonic entities fighting an hypothetical war, and propose the north to take Terran. I can't find a reason for that.
Let's forget about the bunch of rogue regions in the north that have been unruled for quite some time. Now let's think that many nobles in Terran (I don't know exactly how many, but Hireshmont II amongst them) follow the SA. I think they have more reasons to let hem be (I won't talk about contributions), than make war against them.
Now, Aurvandil forbids the spreading of any religion within its borders, in quite a particular way that maybe they like I guess to reaffirm their peculiarity comparing the rest of Dwilight (I don't know what all its regions are like, but the northern marches follow several religions, SA amongst them). They have given home to Allison, a character who decided to "apostatize" from the SA because she was accused to bribe in I don't know exactly which process. She even "threatened" the elders who gave her a hand of redemption, assuring she would found the "true religion" SA should be. And then she called it "Orthodox Astroism" or something similar, in what seems an exercise of provocation.
Personally I think Theocracies would have more reasons to be willing to fight Aurvandil and/or Allison than Terran.
I assume you think Aurvandil should take Barca to give you some parcels to found your own realm. Or maybe they find it necessary because they can afford it militarily speaking, I don't know.
And Terran could perfectly combine its existence with the possibility for the northern realms to bring war to Aurvandil, the way it's been mentioned.