Cooling gas are leeking from compressor safty valve. What are the causes?

could be a bad compressor but I think not
unless the system was properly vacuumed down before adding the gas, it is probably icing up the TX valve and causing a blockage
If you got the correct compressor for the gas you used is another question
feel the suction line to the compressor and it should feel cold when running as that is a low pressure line and the gas will be boiling off in the vacuum and cooling the line
check the high pressure line and it should be hot to the touch as it is compressing gas and that heats the pipe up
if that is not happening then it is the compressor

I am concerned that what you have been told may not be exactly true. If the pipe to the inside of the car from the ac compressor is frosting up ( water condensation) then it has a low pressure area which indicates that the gas is getting through the evaporator ( bit inside the car). If the evaporator was blocked then the low pressure switch would switch off the ac compressor (low gas/no gas)
because the gas could not get past the blockage then the high pressure switch would stop the compressor because of high head pressures which can damage the compressor.
The most logical thing is that the TX valve is partially blocked or the thermostat setting is wrong.preventing sufficient gas flow into the evaporator to cool the car.
If you car has a climate control system then that unit may need checking. Best have an accredited ac specialist shop check the system
And to answer you question as to possibility of flushing or cleaning or washing out the evaporator. this is not a possibility as there can be no moisture what ever inside an ac system. However you can check if the cooling fins on the evaporator are blocked with an oily type dust then you can use an ac fin cleaner . This will allow the air to pass through the evaporator fins and help boil off the gas from the TX valve.

May need to change the orifice valve, or if you mean the compressor cycles on and off, pressure switch is likely culprit.

You may have an air pocket with no fluid. This would cause over heating. If you have a recovery tank, it is better to add fluid thru there and not directly in radiator neck.

No . it will not affect the cooling. If the compressor was replaced because of internal failure it is possible that there is metal stuck in the TX valve orifice preventing full fluid flow to allow the evaporator to work properly. Basically how an ac system works is this . There is gas in the system. A compressor turns this gas to a liquid in the condenser. This is achieved by the TX valve restricting the flow to the evaporator ( bit under the dash). As there is a low pressure in that unit from the compressor trying to suck the gas out then the liquid is converted back to gas ( boiling off quickly from the heat in the car) which cools the air around the evaporator . The process is repeated . IF the tx valve becomes blocked or overloaded by excessive compressor oil then you have problems with cooling.. Have it checked out by an accredited ac specialist shop as the differating pressures between the suction and high pressure sides of the compressor will reveal why it is not cooling.

This problem can be caused from a malfunctioning expansion valve. This valve is what meters the amount of refrigerant that enters the evaporator as a gas. If this valve fails to go the proper position for refrigerant line temperature, it doesn't convert the refrigerant into a gas, but instead it remains a liquid, and the cooling effect stops. You need to take it to an AC shop so they can check and see if the expansion valve is working properly.

you always recharge on the suction side near the compressor. wear safty goggles!!!!This is the largest line and it will have a pin valve on the line for charging.Look for a sight glass ,it will be on the high pressure line near the dryer assy,a much smaller hose-line it will also have a pin type valve like a tire valve,the site glass is used to see if you are near full charge while recharging,buy observing bubbles.. if the unit is cooling the low side or line for charging will be cold,dripping with condensation on a hot day,and frosty on a cold day.if the system is low it will be warm and dry. but if it is low enough to be less than 50 psi the compressor will not opperate.

if the gas was low, you might have a leek
when there is not enough gas in the system , the ac compressor will not ingage

vacuum leaks can cause all the problems your describing and needs to be repaired

if you by passed the relay and clutch engaged then this shows that pump works ??but was it getting cold on and around the restrictor valve where the pipes enter the cab is normally a valve block but american monsters use a valve in the pipe for cooling and if its not getting cold their or blowing cold inside cab then system needs re-gassing with R 134 gas if its an older system with R 12 gas then it will need to have adaptors fitted to valve connections and refilled with R134 gas.The main cause of A/C system loosing gas is porosity in the rubber flexible pipes also the back seal on the pump.Or just regass and put a leak stopper in the system