Duke William of Normandy won the battle because was well prepared and had a good army. They prepared carefully for the battle. The Normans had knights on horseback who were skilful fighters.
A utility knife, metal ruler, and a hammer can be faster than trying to carefully saw an opening. Score the drywall a couple times till you are most of the way through, then just knock a hole with the hammer and break the drywall back at the score lines.
Cut a piece of drywall 1/4-inch short to fit between the corner and the last installed panel. Screw the drywall in place. Where the panels meet, cut a V-groove into the short untapered ends with a utility knife, as this will make hiding the joint easier when you're taping.