Unless its`s possible to set some sort of safe area that reverses Z-axis in mach3?

Wiring does sound like the suspect, as price of copper has increased so has the thickness of the insualtion and thinness of conductor on some Chinese gear, looks like the right wire until strip it open.

Straightforward to test with multimeter between connectors of controller and stepper connector, give everything a good waggle and roll the axis by hand, look for open and short between wires.

let us know how you get on.