Ramsay wrote the letter. Everything written he (Ramsay) believes to be true. The reveal will be that Stannis and the Manderly's met up in the field and sh!t stomped the Frey army. Then the Manderly's went back and gave a false report to Ramsay about Stannis being dead (with heavy Frey casualties conveniently) but Theon escaping to Castle Black with Jeyne. This will cause Ramsay to drop his guard at Winterfell and allow Stannis to sneak up and take Winterfell like Theon did (since Stannis will have gotten that info from him). Then Stannis will sneak in to Winterfell with the help of the Manderlys and Umbers on the inside and the whoop ass will commence.
This is all leading up to the big demonstration of Stannis' military prowess that's been built up over the previous novels. TWOW will reveal the time lapse of events that we've missed that lead to Ramsay sending the letter to Jon where we'll get the Theon interrogation, the Manderly doublecross, etc.
The most contrived moment award for me is being held for a potential reveal of both Jon and Tyrion both being secret Targs. I'm fine with a plot twist of one of them being a Targ, but both just strikes me as unimaginative. For me if they're both Targs it undermines the story arc uniqueness for both of them.
I don't see Stannis making it out of the north. I see him routing the Boltons/Freys at Winterfell being his highlight. Then while planning his moves south the north will get hit by the others and Stannis will be the major casualty of that first large scale clash. This establishes the others as the central antagonist faction going forward and completes the hinted at Stannis story arc revolving around him not in fact being AA reborn.
In terms of magnitute I vote for the Red Wedding, hands down. Every region of westeros has varying views on things but the law of hospitality seems universal. You add to that the fact that you're betraying your sworn allies (and king in this case)... and at a wedding no less... no contest, it's wrong on every possible level in every possible region/culture.
In terms of stomach turning gruesomeness? There's lots of contenders, most of which have been already stated. Little surprised no one has mentioned the R'hollr sacrifices though (unless I missed it). Being burned alive is a horrible way to go. I don't know that it would get my final vote but I'd certainly give it consideration.