Flying in formation is a military skill, If the pilot or co-pilot were former military I could see it happening.
As far as the radar is concerned, Most of the current ‘radar’ systems in used to track aircraft are actually transponder systems. A beam from the air traffic control ‘radar’ activates the transponder to cause it to transmit a IFF signal in return. The reason for doing that is obvious, it makes the automation of air traffic control much easier as each flight and aircraft is self-identified.
It would be interesting to see what kind of flights the pilot was doing on his home simulator.
Yes it takes practice. The biggest problem with the theory is that at night, it would be very hard to track down the singapore jet; especially with the transponder turned off. With the transponder on they could track the singapore jet with TCAS up to about 30 miles. With the transponder turned off, it would not be so easy.
Several things point to a military person being in the cockpit. Someone 'trained' by Americans - maybe from Saudi Arabia? A Saudi - flying as a courtesy - sitting in the jump seat for free and not listed as a passenger?