I don’t know about the wishlist, best to ask amazon themselves, but about the gift-certificates…
Redeeming a certificate does not mean you are “using” it, the amount stays in your account until you purchase an item from amazon. Then you have the opportunity to use your gift-certificate balance as payment for said item.
Redeem them immediately and use them whenever you buy something. Correct me if I’m wrong, everyone, but I believe the amount will stay there forever, or as forever as a website gets, until you use it.
Once you redeem a certificate it can never be un-redeemed or taken back by the person who bought it.