I suggest you try to test to ground at the contactor once again , with the new meter . maybe disconnnect the crankcase heater totally and try to restart the unit.
Maybe only have the compressor in the circuit and try to restart.
Maybe try to replace breaker that trips , just in case of a bad breaker.
Last option . buy a new unit...
I am not sure how to test the contqctor to grund, but I isolated it by removing all wires connected to terminals and probed each terminal to the grounding lug. No resistance. Measuring from one coil terminal to the other, I got 9 ohms. If I measure from the load side to the grounding lug I get no resistance. The contacts are burnt but they open and close. Even when the CB pops the contacts remain closed until the thermostat control is turned from cool to off.
I disconnected the crankcase heater. I tried this earlier and the results were the same. No difference.
Earlier I tried swapping the load lines from the 50 A breaker to the next largest one I had on the board, a 40, and it made no differnce. Still, it was possible that a good 50 might handle what a good 40 could not and I got one from the HD. It still popped with a new 50 Amp.
I ascertained by trial and error that it only pops if both the Black wire from L-1 is connected to the common terminal of the compressor and the Red wire from L-2 is connected to the Run compressor terminal. If either is disconnected from their respective contacter load terminals, nothing pops. I take that to mean that the short is in the run winding.
The circuit breaker opens if the yellow lead to the start winding is disconnected, but common and run terminals are still connected. If the yellow is connected but either the common or the run terminal is disconnected the cb still does not open. I may be misinterpreting this, but this suggests to me that the start winding is good but the run winding is shorted.
One thing that I don't understand is why I don't hear a humm from the start winding trying to do something, even if by itself it cannot start a compressor. Perhpas there is a delay before the start winding kicks in and the thing shorts before that can happen. It is immediate.
As for replacing it, I contacted a 4th contractor, actually Home Depot who will have one of its contractors call me.