Question

If the key doesn't work, the winner has two choices.

They can either tell you and you both agree to cancel the giveaway - the winner gets no game, and you get no CV. No penalty.

The winner marks it as not received. You get a penalty on your account, and a suspension if it keeps happening (not sure if you get suspended on the first time). The winner hasn't done anything "wrong" because you're not supposed to make giveaways where you're not sure if the key works.

However, you might be interested in this group. It is specifically for keys that aren't guaranteed to work, and every member is expected to agree to cancel a giveaway if it happens. I don't know if they accept new entries but it won't hurt to request to join.

The worst you will get is them leaving "key not received" feedback, but you can request their permission to delete the giveaway so they don't have to leave feedback at all if there is an issue with the key.

That having been said I would not suggest using this particular key for a giveaway that you've already given away once and don't necessarily have the correct details about. Just give away some other keys, if you have any.