[68C15]

judging by how you have lower vent temp with fan on low it sounds like a blend door problem.
for the condensor airflow issue you could put a 12"fan in front of condensor or put a plate covering gap @ top & bottom. the extra fan would be better. use the compressor clutch circuit to activate a relay.
optimum pressures for a 75-80 deg. day in sun with fan on high would be low about 30-35 psi, high about 150-175 psi. should have near 20 degree drop in temp across condensor & 5 degree drop across evaporator. I do caution about using the 12 oz cans. while switching cans you could get air in system . I suggest having a good AC shop do the evac & recharge with a machine. R134 is more finicky. R134 usually takes about 75% charge as compared to R12.