Looking at the 3 charts (post 18), those are for refrigerant cans with a low side gauge and are for "accumulator systems". If an expansion valve system is charged to 45-55 psi low side, then the system is way overcharged.
Looking at OP's numbers, I think 40 def F superheat is way too much. That tells me anyway the evaporator is starved of liquid refrigerant. I would try to find the coldest spot on the low side pipe near the expansion valve and double check.
There are no specs for supercool on the condenser side, but hvac industry looks for something like 10 F. On the condenser side 23 F supercool I would say is ok and probably slightly high (as well as the pressure).
OP can try running this setup for a while if the evap is down to 4 F (is that at idle?). My concern would be at higher rpms the high side might go through the roof and cause pressure relief of the refrigerant.
I do not know about flap position readings. I wonder if clamping off a heater hose can help test hot air leaks into the cold side.
Just want to make sure, you are measuring superheat temperature on the bigger pipe coming out of the expansion valve correct? If the subcooling decreased on the high side and if the low side temperature probe is on the >>liquid line<<, that would explain the higher "superheat" next to the expansion valve by roughly the same amount.
Also, is there frost on or near the expansion valve?
Additional info video: