A: Because I own Ashe, the trigger (for my enemy's card) is my responsibility. Therefore, it is a Missed Trigger penalty and because it was advantageous, I would be dealt a Warning. Because the trigger was missed from a previous turn, we would continue play (TR 702.2.f.1).
Few things to note (about this obnoxiousness):
- My enemy would never receive a FMGS or Angle Shooting warning as they were not responsible for the trigger. An argument could be made that the enemy is trying to fish a warning for the opponent (angle shooting), but that would require an investigation.
- If my enemy holds again later in the game, it would trigger Ashe (as it is a delayed trigger) and there would be another opportunity to restore the banished card back to the hand.
I would advise many Yellow deck players that when using Ashe, put their banished card tucked behind your legend so you have a physical reminder to return the card back to them when they hold.
I personally think this rule needs a revision, but I'm not exactly sure of a good suggestion.