After reviewing the primary radar recording, the turn back was most likely at a standard 25 degree angle of bank left turn.
Failing someone for diverting to the NEAREST SUITABLE AIRPORT seems harsh.
The loss of transponder, ACARS, radio communications, Flight ID, and diverting to an airport is consistent with an onboard emergency.
TRE's such as Zaharie teach and check to the principles of ANC. It is the gold standard of how to handle a major inflight emergency. So the first reaction to a major problem is to sit on your hands for a while and make sure you ascertain exactly what is happening before any precipitate action.
We know that MH370 had both engines operating normally, a fuel system operating normally, a flight control system operating normally, a navigation system operating normally, a Satcom system operating normally; which included means to communicate any emergency should all other comms be down.
So the turn towards Malaysia, if you really believe Penang was the goal; (it wasn't), would still take a few minutes at the very minimum to happen and would not be the very first response. Penang also had a curfew which the Captain would be aware of. You also haven't explained why they didn't land there but managed to fly for more 6 hours in a straight line to the SIO which can only be accomplished with an operating and programmed FMC (LNAV) or an operating and correctly set Autoflight system. (True TRK)
Q. Hamid was the PF. Who flew the initial turn and how do you know?