[EVERYTHING] 3D Chess with Petyr Baelish

Ever since Maester Wolkan mentioned that all letters sent to Winterfell are archived, I've seen people talking about a letter that would supposedly be within those archives that would reveal Littlefinger's betrayal of the Starks. I don't remember which letter that would be, but if such a letter exists, Baelish would be doing everything in his power to obtain it and keep it from being discovered by anyone else. This must be the letter that Maester Wolkan handed Baelish in the last episode and the letter Arya found was a ploy. We know Baelish suspected that he was being watched, he wouldn't have stuck around in the shadows after leaving his chambers otherwise. The letter that Arya found was Sansa's letter to Winterfell just before the execution of Ned Stark, it had nothing to do with Littlefinger. Using it as a ploy would not only throw anyone who would have been following him off of his trail, it also strongly reinforces Littlefinger's claim that he loves Sansa and would do anything to protect her.

I've seen a lot of people in the other relevant thread suggesting that Littlefinger did this to create tension between Sansa and Arya. Judging by the way Arya grilled Sansa, it seems like there might be something to this theory. I just don't understand why Arya wouldn't have known that this letter was written under duress. She went to King's Landing with Sansa, she knew Sansa was arranged to be married to Joffrey, and she had to have known Sansa was taken prisoner along with Ned. She had to hide her identity and pretend to be a boy for this very reason.

Come on, Bran. You're supposed to know everything.

Edit: Furthermore- Catelyn, Robb, and Maester Luwin figured out that the letter was Cersei's doing immediately. Cately was the only one of them that had ever even spoken to Cersei. Sansa is the only remaining Stark that knows Cersei better than Arya, she should have no trouble determining that Sansa was forced to write that letter.