I beleive I have read every post.
Has it been verified that one of the last contacts signed off with the words "Roger that"? If so, then the investigators need to question as many crew as possible that flew with the Captain and FO to ascertain if they ever used those words. IF NOT, then I would investigate the individual who had flight sim training and see if he was taught or used these words in training. The answers to the above would point to either Southern route an Captain/FO surcide, or Northern route and hijacking.
The individual who had flight sim training intriques me. As a SLF, I had 2 separate one hour sessions in a major airline's 777 SIM. I found that controlling MASS is no easy thing. Some of the reported altitude excursions could be explained by someone like me thinking the real plane flew like the SIM only to find out that it was massively more difficulut in a real 777 for an amateur. The amateur would then switch to VNAV and LNAV and other modes to let the computers do the work.
Just thinking out loud......