Now why would anyone NOT use LNAV? Because it’s not available at that time.
Not necessarily. The pilot might well have turned back in HDG mode, then engaged LNAV once he had determined the correct waypoint to enter in the FMC. That waypoint would not have been in the existing flight plan. HDG mode is often used for short-term manoeuvring, so it’s hardly unusual.
Even if LNAV wasn’t engaged until much later, so what? It doesn’t necessarily mean it wasn’t available, as you claim. The prevailing theory is that the pilot had no intention of landing at Penang, or anywhere else. You don’t need LNAV for that.