Steve
I looked at your XML and could see no issues regarding probe input.
While I had it loaded I tried a G31 move and it stopped when the probe input was emulated.
I don't know what else to suggest apart from try a clean reinstall of Mach.
Greg
EDIT: Just had a thought.

If you are testing on a computer without the driver installed then emulation of the probe input will not work.
Do you have the driver installed?