"If MH379 was operating in LNAV which seems to have been agreed"
Nowhere has it "been agreed", nor should it be. As I mentioned in an earlier post, don't get mesmerised by the FMC, a very useful bit of kit for flying a pre programmed route, doing a SID or making an arrival.
The aircraft is believed to have flown a random route, devoid of known waypoints, from the time it went back over Malaysia until 08.11. If the aircraft was under the control of a person then FLCH and HDG Select are the most likely modes used.