Sachys: Ungrateful swines!
What was it?
So there's one last twist to the tale...
I PM'd support saying I'd had a key nicked (told the whole story, including the fact that I was giving it away) and GOG support, being the cool bastards they are, checked it out.
The account that had redeemed it (they didn't tell me the username and I don't blame them) had indeed brute-forced the code. Since my story checked out (and I had proof that the code was originally mine) they removed it from the scumbag's account and gifted me a fresh copy to give to the rightful winner of the code. I can't think of another DD site that would do this.
tl;dr: Game was taken from the scumbag, went to rightful winner. GOG support is full of cool guys.