The location of the possible debris does not match the flightpath expected to have been taken by the jet, which should have flown directly over Ho Chi Minh City. The location supplied to the Hong Kong’s Civil Aviation Department was more than 125 kilometres southeast of the original flight path.

allowing a plane at least an engine and/or some hydraulics to wing and/or tail?

Passenger jets are not great gliders, but they do glide. Does it seem plausible to anyone that a plane could suffer an explosion that resulted in an immediate loss of communications, but allowed some limping along (probably without much ability to turn) for a while before finally diving into the sea?

If the autopilot is engaged, there may be enough of the actuator control electronics, the primary flight computer, the inertial platform and the other attitude and speed instruments connected to allow the plane to fly on for some time.

Without a transponder or communications, it should show up as a blip on radar due to passive reflection. However, the signal is much lower than with a transponder.

Not sure where everything is located. But a bomb in the cockpit might wipe out the crew and coms without disabling the PFC, etc. Some of the stuff seems to be distributed spatially to reduce common failure risk.