Headphone out for live playing?

Have you tried hooking up to the TS main outs instead?

Are you panning hard left and right into 2 channels? You need a stereo cable if you run out of your headphone jack.

In theory yes you can but I would try and find the source of the underlying issue instead. It sounds like you may be overloading the desk mic pre when using XLR. Try TRS instead and see if that helps.