She even puts a protective blessing on the party beforehand. So yeah, I think she should be marked green ally. Although now that I think of it, we've had yellow-marked allies before (gnomes).
This said, I do like the change of pace with this encounter. For once the enemy comes to the party, not the other way round. And there is wiggle room for maneuvering during late dialogue.
Overall, I'm inclined to be satisfied with this encounter as is. Yes, even with clever mitigation, you can still get smacked by the whim of the RNG - with big repercussions and high emotion!
But come on, this is Kethric and Balthazar's doing, and they're not chumps. (Kethric's my favourite bad guy btw.) Them pulling off a devastating attack sets the scene.
If anything, I'd like a second attempt from Z'rell if Marcus fails. But I guess by then moving the amassed army onto Baldur's Gate is priority #1.
