So far homing seems to work. I (from memory, not in front of the machine as I post) set several GRBL parameters -- turn on homing, hard limits, soft limits enabled and specified. Also changed the values for table size in Carbide Motion under Settings, and turned on the homing switches for Shapeoko there. I used 845 for max X and 925 for max Y I believe (both in GRBL and in the $1000-whatever parameters in CM's Settings tab). Default position after CM homing is just a bit to the left of zero, I haven't bothered to figure out where to change that -- it is probably obvious but not a priority.
I have not yet calibrated or even properly squared the machine, but CM seems to work just like on my Nomad now. Homes automatically when you first select "Jog", fast moves to all the various positions with no problem, etc. Ran Hello World again successfully.
Homing itself seems to "give up" significantly more than 1mm on each axis, more like 5mm or so by eyeball,