So what if it's potentially violent?
This:
More likely, we'll just be forced to orchestrate a peaceful religious takeover of either regions.
You cannot have a "peaceful violent takeover". An auto da fe is a forceful, violent act. It is not, and cannot be, "peaceful". That's like saying you're going to have a "peaceful war".
So in other words, the only way for D'hara to get the region if Terran gets it is for a noble of Terran who doesn't want to join D'hara, must join D'hara by changing allegiance...
Hell if I know. Ask Tom. He's the one that made the system. For my part, if Terran doesn't want to keep the region, they shouldn't TO it. Or they should take it from Provincia de Flor-something to kill the realm, then let it go rogue, and then let D'Hara take it. Or find someone from D'Hara to go to Terran, get appointed lord, and bring it back to D'Hara. RP some oath or other that will let you do that, if you feel it necessary.