Deleted

What are the repercussions if they don't work?

The winner will have to mark the giveaway as "Not Received", or they can give you their permission to delete the giveaway. In both cases you lose a giveaway slot, so the only real difference is if it's visible on your profile or not.
You also gain an extra slot for every 3 gifts sent by the way.

is it possible for keys to be revoked if they aren't used after a certain amount of time?

The vast majority of the time no, the very few times a key would expire, it's usually explicitly mentioned.

There doesn't seem to be a way for me to check if the keys are still valid without adding the games to my account...

There is no way indeed