Hi! New to the forum and literally just finished reading aDwD about three hours ago. Then went to wash dishes, and independently came up with the "Jon is not Jon" theory. Then Googled, and got here.
Wanted to add something that I think is under-touched upon: Jon is *really* not like himself in his last chapter, especially when getting the letter. A number of his anecdotes ("as we say," etc) are, unless I'm remembering very wrong, unlike anything he's ever said before. And his reaction to the letter is totally out of character based on everything we've seen since he took the Black.
I can't figure out exactly how the Body Switch would take place if Jon is not Jon, and to me that seems a little too...convenient. Nevertheless, it's very plausible and I think at this point it's the most likely situation.
But I was reminded of another fairly recent surprise fantasy-series death: Dumbledore. The death, which seemed like a super-horrible ambush/betrayal/murder was anything but. And I think something like this is in play here, too. Jon is very clearly not himself (at least in terms of attitude) in that chapter. And why cry if you're doing something that you feel is just, from the perspective of Bowen Marsh? If Jon really ruined the Night's Watch that badly, you should be elated to stab him.
Of course, if there's some collusion between Marsh/other assailants/the Giant distraction/etc...that informs the entire thing way better.
Finally (to wrap up a long first post,) there's no evidence that anything in the letter came to pass -- and the only person with knowledge of everything in the letter (6 spearwives, etc. etc.) would be Jon himself*. Regardless of whether Jon was in on his own stabbing (and to what degree,) the letter does not fit with anything else in either aDwD or the preview we've seen of tWoW. So the letter, to me, seems a plant by Jon or someone extremely close to him, and not authentically Ramsay Snow.
*This does not explain the Reek reference, but everything else seems to fit. Again, I only wrapped the book a couple of hours ago, so those more read and re-read than I may be able to flesh out more detail.