An irrivocable credit is "irrevocable".
Unless all parties agree to cancel the obligation it cannot be cancelled. What happens if your customer is trying to pull a fast one on the vendor? The bank would be liable for any and all funds. If the vendor agrees to cancel, have them return the original credit to you, along with their written authorization.
#2 - IMHO the same no answer - it becomes a liability of the estate, unless state law says otherwise.
_________________________
Integrity. With it, nothing else matters. Without it, nothing else matters.