There was an easier on the spot solution. If you're going to be executed for attacking a noble, then you don't have anything to lose.
And there's a subjugation ring right there that can bind someone to you on pain of suffering. Just bind the Chief Priest and have him kill the noble.
Now he's on the hook instead of Myne and can't fight back because of the subjugation ring. Have him defend himself in court - that the noble attacked with his allies and he and defended himself without knowing the noble was there.
Bam, job done, case closed, only bad guys suffer.
It's not that easy. The High Bishop (evil santa) is of a very high status, given his family ties. So binding him with a subjugation ring is by itself an offense punishable by death. And the other noble is from a foreign and higher status duchy. So it is an interduchy incident.
You need to have absolute proofs of their wrongdoings if you want to act against them, otherwise any actions could be seen as a declaration of war. Having the High Bishop kill the foreign noble would be considered as Ehrenfest declaring war on the foreign duchy (Ahrensbach), and that would be extremely bad. Ehrenfest would have no chance at all to win in that situation. So they would instead pass all the blame on the reckless commoner who dared to fight against the nobility, and they would execute Myne and all of her family. Even then, that wouldn't be enough to avoid other bad repercussions for Ehrenfest archducal family.
Besides, how would the High Bishop even kill the foreign noble? One is able to use magic, while the other can't. As a true noble, the foreign noble also likely has some protection charms that would prevent easy assassinations like a dagger stab. Enough to give him time to blast the High Bishop to smitherens if he wanted to.
