I guess I will share my theory for how the Undead Nightmare story will be explained.
It will contain MASSIVE ENDGAME SPOILERS
First off, the description for the Undead Nightmare pack, as seen on the RDR Official Site:
Unleashed upon the world of Red Dead Redemption, the Undead Nightmare Pack will feature an entirely new mostly single-player campaign that follows John Marston as he tries to find a cure for the highly infectious zombie plague that has spread across the frontier.
Now, like a lot of people, at first I was kind of worried that this would tarnish the integrity of the Red Dead Redemption universe. But then I saw this on the most recent "Asked and Answered" on the Rockstar Newswire:
We love the world of Red Dead Redemption and wouldn’t do anything to sully it. While we respect your doubts, we ask you to trust us – when you play the game, you will understand what we have done and, we hope, why.
They didn't just outright say "it's set in an alternate universe", they seem to have some sort of secret they're holding about it. Now I thought about it for a bit, and came up with this.
What if after John's death, and after Jack gets revenge on Edgar Ross, he actually DOES become a writer. And what if "Undead Nightmare" is his first novel? He writes a fictional story , with the protagonist being John.
What do you guys think?