I’m no super pilot but I don’t know any case that forgetting the loudspeaker had resulted in a loss of communication.
Don't bet on it. Jet Airways has done it twice in European air space. On the second occasion over Germany fighters were scrambled. But it shouldn't have happened. It is matter of doing it in procedural way. Increase the volume First then remove headset. Off course Jet airways is Murphy's airline. If any one can do it then they have done it.