I agree I think that option was only there to make us hate Vaylin and perhaps to remove any sympathy people got for her from the previous chapter. Speaking for myself it didn't eliminate my views on Vaylin, if anything looking back at that chapter, I blame the Outlander for the deaths of Torian / Vette. The player was the one who listened to Valkorion then baited Vaylin to Odessen to join the battle.
As you say though, the decision to save only one was a stupid decision, you have two companions with you at the time of the decision, also having passed four or five alert companions and generic alliance npcs. I wish thee had been an option to save both and Vaylin depending on how many Alliance Alerts the character had completed.
Not sure how to describe what the Inquisitor does if you attack Ashara. Mine just stood there as if his mind just went dead while she turned her back to him.