Even if one of the oil slicks did prove to have come from the aircraft, it still does not mean the aircraft crashed there.
In the event of total engine failure, surely the pilots would have dumped their fuel to reduce weight and improve gliding performance?
On another point, perhaps the various communications antenna on a 777 (or routing of the cables to them) are located in such a way that a single event could knock out all communications, radar transponder etc. together.
Assuming that the same event knocked out the engine controls and the navigation lights, the pilots would attempt to glide to the nearest suitable runway, which would probably be KBR (Sultan Ismail Petra Airport).
Assuming the eyewitness report is accurate then the navigation lights were disabled in the unknown incident but the landing lights were still operable and they were turned on. The position of the sighting is consistent with the aircraft trying to line up seaward of Bachok for a landing at KBR.
If the gliding aircraft ran out of altitude before reaching the airport it could have ditched at low speed resulting in no break up and therefore little or no release of debris. If the fuel had already been dumped there would be no oil slick either.
Damage from either the ditching or the earlier incident could have caused it to sink rapidly. Maybe even some bags containing mobile phones floated free.
Speculation but seems plausible. I'm no expert - any of the experts here think such a scenario is feasible?