Wow. If I'm spending that much money I would likely want my AMEX protections then a discounted gift card. But it's still good to know. How long before the GC expires?
Not if you're buying consumables/perishables.
I'd guess the giftcards don't expire, but I'm not sure. You can add them to your account and they'll be there for future purchases, haven't seen an expiration date.