-The anti-merger rule is meant to preclude peaceful merger. There is nothing peaceful about the current situation. Lords of Terran are being thoroughly beaten, and they try to find a way to keep their lands. I think that's entirely legit.
This is absolutely a peaceful/friendly realm merger. If Terran were surrendering to the realms that are defeating it in battle, this would be acceptable. It is not. Terran is merging with a realm with which it has peaceful relations. This is not a surrender. Desperation move, yes, but only so as to deny its regions to realms it dislikes more than D'Hara. That is definitely not the intent of the rule.