We finally figured out a way to line up in the north west corner of the board. We built a new much sturdier plow with wheels on the front corners. We use a wheel and careful driving to line up our right side against the north wall, then we run into the west wall (using move blocks set to run for "time") lining up the front of the robot by doing so. We were also able to eliminate mission 4, release the red truck, by simply releasing it as we drive past on our way back to base.
Also new is how we get back after releasing the truck. We now measure the distance we travel before hitting the black line after releasing the truck. Based on that distance we can calculate the amount of turn we need to then go straight to the south wall, at which point it is easy to get back to base.