Hmm. Obviously negative situation.
Confort or full Premier (or did I miss that?)
For comparison, a STE+ I happen to know (not me) lost his immigration card a while back. Was allowed to proceed to the gate (hey detractors, don't know if you're familiar with Mexico but the check-in agent is supposed to check, and if you don't have one, issue another + fine).
At gate, they didn't want to allow boarding. He threatened to call the embassy from there. He was then allowed to board.
Otherwise yes, AmEx is worth many times its weight in Gold.