Not an expert on any of this, but...

Costco may stand behind them. If they don't, might you have recourse via American Express, assuming you used that to buy the cards?

If you are holding them now, could you use them to purchase a fully refundable ticket for 11 months from now? Would having the value in a paid ticket be safer than holding it in a gift card?

As far as future gift card purchases, the discount is 10%. So if you think there's even a 20% chance that you'll end up with worthless cards, then don't buy.