All of these questions have been answered many times in this forum, but in brief...

Plane to city without waiting for baggage is 50-80 minutes depending on where in the city and how long you have to wait for a train (every 12 minutes).

Getting a train from Hong Kong station 90 minutes before scheduled departure time is comfortable. You could squeeze that a bit if you have your boarding card, and if the gate is close to security.

Immigration is a normal line up inbound and outbound. Waits can occasionally be up to 20 minutes, but normally around 10 minutes. Customs is a simple red/green channel walk through.

You should comfortably get 5 hours in the city; you could probably squeeze it to 6 if you have to.