how do my wife & I get from Hong Kong airport to The Kowloon Hotel late at night? We land at 11:15pm

The last bus of A21 is at 00:00, so I think you can still catch this bus line to reach Middle Road, Nathan Road Station. Upon reaching there, you can walk to your hotel easily.
If you cannot catch the A21, you may choose to catch the line N21 to Middle Road, Nathan Road and then walk to your hotel.