100% agree do not make any plan with any Airport Express station other than the HK station due to the uncertainty.
Also dont count on the MTR stations open. The services generally stop at 10pm but stations at hot spots especially during weekends, can be closed a good part of the day or suddenly close.
I dont know a 7 hours transit these days are worthwhile to head out to town and then back - because
1) the transportation can be iffy. if several MTR stations closed and then you need to take the A bus, then you will be subj to road traffic.
2) you may face a long queue to enter the airport departure hall upon return. Only people have same day departure proof and proper ID can enter - but the checking is done manually. So it depends on what time of the day, for how long the line / wait could be.
On our recent departure of a 9:50 flight, we arrived the entrance about 7:45, still had to wait about 10 min to finally get inside even though I could see there were no more than 10 people waiting to go thru.