You can get quick release adaptors with integral shut-off valves. I dont lose any refrigerant when disconnecting.
If you invert the cylinder then it is liquid up to the handwheel valves on the gauge set, it will vapourise very rapidly between the valve block and the vehicle service ports.
I prefer to 'liquid' fill the first 300-400 grammes into the high side, especially if adding oil. I then start the engine and add the remaining refrigerant through the low side but I make sure that the LP handwheel is only just cracked open to ensure that only vapour reaches the pump.
I use this method because I like to monitor the system pressures as it fills. I am looking for a stuck pressure regulator in the pump, a blocked condenser or a stuck expansion valve and have seen all on Omegas.
You should never try to fill via the high side if the engine is running, the pressures are too high and you risk backflow into your bottle and contamination of your nice clean refrigerant.