There has been some confusion concerning WOTN Arya & Wildfire Assault. If I have 5 Stark characters on my board and I choose to play Wildfire Assault, I select 3 characters to save, one of which is NOT Arya--thus, she and another Stark character are going to die from Wildfire Assault, can I then trigger her ability after the first character I control is killed or do they have to die simultaneously?

Also, if I am going first, both players choose their 3 characters to save, if Arya's ability hits with Wildfire Assault and I choose one of their "saved" characters, can my opponent then decide to choose another character to save instead? Or, do I choose my characters, kill them, then my opponent chooses his/her's?

Wildfire kills all characters simultaneously - not one at a time.

As both Arya and the 5th character would be in the dead pile by the time you react to deaths caused by Wildfire, Arya couldn't trigger unless you chose her as one of the targets not affected by Wildfire.

If you DO choose to keep Arya on the board, here's the detailed breakdown

Wildfire's When Revealed:
FP chooses up to 3 characters
Second player chooses up to 3 characters

Wildfire's Kill Initiates on all characters not chosen.

No opportunity to save ("would be killed") because Wildfire is cannot be saved.
Opportunities, in player order, to interrupt the death (Benjen, Shireen, Quentyn)

Wildfire's Kill Resolves on all characters not chosen. They end up the dead pile.

Reactions (like Arya's) in player order to any of the characters dying.