Better look at it when its cooler but I really suspect he stuck a 3 ton unit on 2 tons of ductwork and his lack of airflow which is apparently tolerable in cooling is now comming back to bite him in the butt. I am not sure, without looking but I dont think a 50,000 furnace even has the drive for 3 tons, especially against restricted ductwork.
Now, I'm betting because of low airflow, he has compensated by overcharging the unit. That might explain the HPS trip in heating. But that can be fixed by adjusting the charge seasonally. Add a little in summer and take a little out in winter. Of course defrost will not work well but hey, the cake and eating it too thing comes to mind. Then again you got gas back up.