Diego Garcia: I think it would be very safe to assume that if a large unidentifiable plane flew anywhere remotely close to Diego Garcia it would have been noticed.
Agreed, I would expect the base(s) in the region to be on some escalated level of alert as soon as the plane went missing in "9-11" style transponders off. Like a disturbed wasps nest some interceptors would be up and anchored navy ships put to sea. Based on past events if it showed up on defense radar they would have at least 2 interceptors.
Without doubt this appears to be a hijack given the prevailing data points. The alleged route appears to be have been very well planned.
I totally understand any intentional obfuscation by the Malaysian authorities. It is not in the best interest of authorities to immediately share all information with the public during an ongoing hijack/terrorist incident. It would potentially give too much feedback to perpetrators and enable them to stay one step ahead of the authorities.