The key had not been used - as you can see, I've sent out literally thousands of games and 99%+ have had no problems. If some bundle site or game developer accidentally deactivates or reuses game keys, I have no control or way to resolve the matter, as these are game keys from old bundles.

Frankly, I already apologized, and that should really be enough for you. I am being nice enough to put in a ridiculous amount of time to give away thousands of extra keys that I spent my own money on - as were the owners of the hundreds of other free gifts you've won.

In the future, if the occasional key doesn't work through bundle/dev or even gifter error, you may want to consider gracefully accepting an apology, mark it as not received, and encourage the user to continue gifting anyway.