I made a good-faith effort to undo my victory, and I was unable to do so. Mostly this is because in two out of my three tries, the war never took place. My only surviving save from that period (I usually rely on autosaves and save-upon exits) is from too far before it started. The RNG would not return my falsified claim. And the one time it did, I won the war handily, thanks to the Duke being abandoned by his allies.
Which I guess means my initial failure was a fluke, and thus my attempt to correct it was no more improbably than the event happening in the first place. I still wish I hadn't screwed up and succumbed to the temptation to manipulate the odds with save-scumming, but I rolled the dice three times, and they came up my way. I'm choosing to believe this means it's okay for me to continue with my successful save file.
It's nice being saved from the dustbin of history, but I will always have that asterisk hanging over my accomplishments. However, I have such a long road ahead of me (seriously, I'm less than 1/3rd of the way there, which means another 40 hours at least) that I'll probably come to forget this incident before I'm done.