1) Possible? Yes. Allowed? YMMV.
2) Possible? Yes. Allowed? Not really per the fare rules, but YMMV, I guess. But what would you standby for anyway since you can't change routing? There is only one ORDHKG flight per day on UA. A standby on the domestic portion of an international ticket has always been allowed-- or at least allowed in my personal experience.
This is misinformation.
Standby for earlier or later flights with departure of first segment of the new flight within 24 hours of departure of first segment of the original flight is always allowed, so long as the routing is not changed.
This is true for the next segment in your itinerary at ANY POINT in your itinerary. For example, if you are flying AAA-BBB-CCC (stop) then CCC-DDD-EEE (home), once you land at DDD, you can standby for any DDD-EEE flight leaving any time within the 24 hours before or after your original DDD-EEE segment.
You may have difficulty getting on the standby list without going to the airport.