This is really an interesting theory. However, I don't think Ennis and Lureen had a shared flashback. It is late, so forgive me if I'm not making sense, but why would Lureen know how Jack was "done" if he was murdered? I suppose she could have imagined, but the flashback is very specific, the number of men involved, the tire iron. I suppose the coroner would have figured out what weapon was used to kill Jack if it wasn't found at the scene and Lureen would have been told. But the flashback seems too specific for Lureen to image, IMO. Now Ennis had more of a reference because of what his father told him and what he saw with his own eyes as a boy. Does this make any sense?