A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
If I defend my province with Longji and I use COaaT to duel my opponent's personality, can the other player interrupt with Ward of Air to stop death from the duel?
I think: Longji's challenges are unstoppable, but only if he is targeted by them. Since COaaT creates an ability on Longji, he does not target himself when he uses it, therefore his repeatable challenge ability is not unstoppable.
If I defend my province with Longji and I use COaaT to duel my opponent's personality, can the other player interrupt with Ward of Air to stop death from the duel?
I think: Longji's challenges are unstoppable, but only if he is targeted by them. Since COaaT creates an ability on Longji, he does not target himself when he uses it, therefore his repeatable challenge ability is not unstoppable.
You are correct that Come One at a Time's duel does not target Longji, so it is not given Unstoppable. Also, note that Ward of Air's destruction negation is not an Interrupt, and therefore is not affected by an action being Unstoppable either.