Transport Courtesy China Air

It was anticipated that Kim would use a Chinese jet to travel to Singapore. His sister used Kim's aged Il-72 to go to Singapore. This JTBC graphic shows Kim's route after the China Air 747 picked him up in Pyongyang. As expected the air route stayed inside China as far as it could for security reasons. Kim's cargo aircraft brought his limousine and food supply. It looks like the most vulnerable part of the route was when the aircraft was out of the protection of any air defense forces that could be stationed in Hainan. So on the way back, the most vulnerable portion of the flight will be just after the aircraft takes off and before it once again approaches the Hainan air defense zone. On the way back, they won't be much guessing as to which aircraft Kim will board as people will be watching for his departure.

It was reported that Kim would get Chinese fighter escorts at least for some of the way to Singapore. I haven't seen that confirmed.