What is the village there for? What do they do? Their job and calling in life is to kill mages. That is it. That is what they do. So the idea of any mage running to the defense of the village seems preposterous to me. You are suggesting that Yanni is so bound and determined to kill Glik she is going to save Zah and keep the head of the archmage in the Village? How is that sensible? Would she not have just as much success throwing quicksand at the Empire guard and throwing spells there with her squad of 4? Hell, wasn't Jeunne in that particulat gank squad? So she had a summoner. But rather than let Zah eat dirt and then take Glik later she runs up in the Village to ensure that Zah survives and keeps the head of the archmage. There is no sense in it. Did Zah go right at Yanni after Glik fell or did Yanni get out of that one unscathed?

As a serial mage player I cannot envision a scenario where I like it better when the village has the head.

I don't believe that Zah called Yanni. I believe he called one of the fort assassins who brought the whole gank squad. I personally feel like Yanni should have been willing to let the animals wipe themselves out. Either Glik kills zah and takes the head (so she wins) or Zah kills Glik in defense of the head (so she wins). There is no roleplay justification for her to save Zah's life and keep the head of the archmage in the Village.

No, Zah cannot be held accountable for Yanni showing up. I don't believe he called fer her specifically. But I bet he didn't attack her and drive her out of the village. Zah almost certainly called for help to fight 2 Imperial warriors when it was him and the destructor while he had the head. We can debate on if that is proper until the cows come home. No, I guess it is technically not illegal for mages to help the Village defend against raids and to save the lives of the village. It just seems like a real hard pill to swallow when you sit down and ask "why did she do that?"