Shortly after last radio contact with ATC the crew have taken limited, action. I believe the aircraft has been taken into manual control by the Captain and turned approx 90 degrees away from current fight route with the intent of an emergency descent. One of the pilots have recognised the need to sqwark 7700 but due to hypoxia inadvertently turned off the transponder. The effects of hypoxia have resulted in the crew not donning O2 and the aircraft altitude varying as a result of an attempted emergency descent until they became completely unconscious.
Why manual control? Decompression protocol on 777 leaves autopilot on during descent.
To tune the transponder to 7700 you never touch the on/off switch.
By the way, the altitude warning horn is very loud and reminds you to put on your oxygen mask. It is the 1st step in the checklist and the pilots would not have not tried to turn away, descend or squawk 7700 without doing this. I know this happened in the Helios accident, but we learn from other's mistakes, and they didn't have an EICAS to tell them what was happening.
Your supposition totally depends on both pilots ignoring a blaring warning horn, EICAS messages, master caution warnings, and doing more than one thing against their training, in other words being a totally incompetent crew, which I do not think they were. My two cents.