Even if you have too much refrigerant in the system it shouldn't cause the belt to squeak. That would indicate that the belt is just a bit too loose. Try loosening the 14 mm lock nut and turning the tensioning screw clockwise one turn and retension the locknut. If it still squeaks when the A/C clutch engages with the engine at about 3,000 rpm then you may need another 1/2 to full turn of tension.
Regarding the overinventory of R-134a, first, check the pressure with the engine and system cold in the morning. I have found that 100 psig static (measured at either the high or low pressure tap because the engine is not running) of R-134a at 85F degrees has allowed my retrofitted 1991 R-12 system to put out 33F air from the vents on a 108F day. The static presure will vary proportionaly (temp higher then pressure higher) with temperature, as shown by the scale on the R-134a pressure guage.
If you only have to drop the pressure by 10 pounds or less, I would say that you could probably release it slowly enough so as not to release too much lubricant. Just vent it slowly until the static pressure is about 100psig at 85F.
If you have too much pressure in the system, it will actually reduce the amount of heat the refrigerant can absorb since the low pressure side is at too high a pressure and the pressure drop is not as great as it should be. It is the expansion of the high pressure refrigerant that absorbs the heat in the evaporator. Without the required pressure drop, the rerfigerant cannot absorb the necessary heat and you don't feel as cool.
