I've always been obsessed with the notion that a Reyne of Castamere survived the Lannister obliteration. IMO the most likely character to be a Reyne is Petyr Baelish, because it explains so many of his motives.
I know all about the timeline, believe me. But let's talk about this line in the show:
Littlefinger to Cersei: "I'll not rest til the Lion flies over Winterfell."
This is immediately after Petyr tells Cersei that Sansa is in Winterfell and he is selling her out - and he wants to be Warden of the North in reward.
If Petyr became Warden of the North/Lord of Winterfell, the Lannister Lion would not be the banner that flies, it would be Petyr's banner. If not the mockingbird, why then the lion?