I think massive and longstanding loathing between metamours says something about the "hinge" partner. Why are they OK with being with someone who causes massive pain to someone else they love? Usually one person is in the wrong (at least mainly), so refusing to take "sides" when one side is in the wrong is a cowardly move. I understand not wanting to use veto power but the hinge partner should think very carefully about why they themselves are OK with people they supposedly love causing each other pain. I would feel this way about being friends with two people who hate each other. Unless you're trying to reconcile them idk how that would work.