Putting the boat on the trailer & leaving it in gear to hold it .... how about an extra long piece of cord (or even fishing line) attached to the kill switch lanyard, then pull the kill switch once you get the boat winched up ?? That'd be cheaper than one of those automatic boat latch thingies (Ramp-N-Clamp or Drotto Catch/Release) @ $200-$300

Not sure what to tell you about your launching solo problems ... other than a 50ft ski rope tied to a cleat or bow eye & your vehicle, back the trailer down to the point where you can barely get to the winch & release the boat winch rope, then get in the vehicle and back up & brake to push the boat off and clear of the trailer ... pull up and get out, then pull the boat to the ramp float/dock, then tie it off and go park the vehicle.