I'm honestly failing to see a way for diplomacy to work right now. She's basically won, and even if we somehow talked her down no way in hell am I seeing the others letting her live.
we lose nothing by trying diplomacy before going for the standard plan.
Also it's not really a victory for her when, at the end of the day, everyone still hates her and she has no realistic hope of that ever changing if she continues as she did until now.
The others would probably listen to us. It would at least be easier than convincing Dandeer herself.
There are a lot of good arguments for keeping her alive
1) we can (i assume) safely seal her with Spirit Saiyan fueled magic, so that she can only use her magic when we allow her too (basically we remove-reapply the seal whenever we need her to actually cast spells, or have Dandelor create a more complex seal that allows her to use magic only when explicitely told to by us or one of the other rulers.
2)she's the best saiyan sorcerer bar none. We need better ki wards. Have her work on it, and in a few years we might have ssj2. Until now she had no real reason to research them, as giving the super saiyans more power would only bring her more problems, we can change that
3) She could, under STRICT SURVEILANCE, train other sorcerers. As already said, she knows more about magic that anyone else, with only Dandelor coming close
4) Yammar and Apra are both believers. If we point out to how even Penitent Vegeta was once a monster, they can't really reject this without being hypocrites
5)IF (and i admit it's still a big IF) we succeed, they simply owe us to respect the deal we made. They ALL owe us this much, for more than one reason, from Berra not listening to us to Jaffur owing us both his and Jaron's freedom AND life.