I think trying to do the cruise on Sunday and fly out of Guilin on Sunday evening is pushing it. Consider flying out of Guilin on Monday morning instead--you really only lose a few daylight hours for HKG touring anyway.

Best Monday morning Option: to Shenzhen, CZ3239 09:45-10:45. Check around for pricing but I'm guessing it's not going to be real cheap, maybe you can find for RMB 600-ish, but a flight direct KWL-HKG is probably double that. In the arrival hall area at each Shenzhen terminal A and B, there is a bus counter area where you can buy tickets on a nice a/c bus to HKG (RMB 90). Buses leave about every 20 minutes. The one you want is via new-ish Shenzhen Bay border point which has consolidated all border processing into one building. Bus leaves from right outside each terminal, and there will be a comfortable little inside waiting area with attendant in case you need to wait a few minutes for next bus departure. Attendants won't let you miss the bus. Bottle of drinking water per person provided. It takes about 30 minutes through Shenzhen to get to border point, you hop off bus, grab your luggage from the storage compartment, roll into the building through Chinese Customs and exit immigration, then through HKG entry immigration, then out the other side of the building into a waiting HKG-licensed bus, which take you to the Kowloon Station where you can get taxi or Metro, etc. to your hotel--about another 30 minutes. It's pretty seamless. Total time (done this recently at that time of day) is about 1.5 hours. So you'd get to your HKG hotel around 1 pm. Options through Guangzhou would be more painful due to distance of airport from central city and surface transport locations--don't think you'd get to HKG any faster even on an early-bird KWL-CAN flight.