As a regular flyer between HKG -HGH, the morning flight 7:50 or 8am depends on season is one of the small plane with only 8 J seat.

KA622 has a delay rate of 50% according to my experience, so even though your JFKHKG leg was delayed for 2 hrs, it was too early to rebook you until maybe couple of hours before you landing.

12:15pm flight is A330 and more J seats would be available. To me, it would be a better option as you could rest better in the hotel. I believe that u mentioned abt getting into your room finally at 11:15pm. Go back to the airport before 7am? I wouldn't want to do that.