Since it is a heatpump there is a possibility that the outside coil's check valve has stuck in a "partially open position." It would also could force some refrigerant through the outdoor expansion valve to the condenser coil. In the cooling mode this will result in low suction pressure and low discharge pressure. Sub cooling would be extra high.
If we're speaking about the heat pump operation while in cooling mode, I would think an outdoor check valve that is partially open is actually serving as a liquid line restriction, as in cooling mode this valve should be all the way open, bypassing the outdoor coil TXV.
If the indoor check valve was partially open in cooling mode, it would result in a flooded evaporator and reduced subcooling due to greater mass flow rate of refrigerant in condenser...doesn't have as much time to sit in subcooling loops of outdoor coil and be subcooled. Evap would show a higher suction pressure but low superheat due to flooded condition.
The reversing valve can be ruled out easily because if it was leaking discharge gas to suction, there would be a greater heat rejection load imposed on the outdoor coil, reducing subcooling as well.
I am with those who suspect an indoor airflow problem. Higher than expected head for given OAT of 78 may be due to overcharge...not uncommon to see in heat pumps.
Psychrometrics: the very foundation of HVAC. A comfort troubleshooter's best friend.