Probing: rather than dig around in the electronics in the back of the mill, I just bought another PCI parallel port card ($7 or so), stuffed it in the computer, and connected my touch probe there. Mach3 handles two parallel ports, no problem. This ranks as perhaps the easiest thing I've ever done with the mill; it just works. I have future plans to use output bits of this port to control a pneumatic valve or two, shopvac for a vacuum fixture, etc.
As for the spindle guard: this was removed, along with the coolant tray, within about 10 minutes of mill delivery. In Mach3, Alt-7 - you can see what the parallel port bits are doing in realtime, as shown by the green boxes. Twiddle the interlock switch (or any other switch) to see what bit flips.