All good info, thanks.
The motor is a permanent magnet type and the brushes are perpendicular, not offset. I have tested it on the bench and it runs both ways by reversing the wires.

I thought I would be able to rewrite the M3 and M4 macros to read the actual spindle speed and not reverse until actually stopped. I usually start these queries with an admission that I am an electrical idiot. Should have done that this time too.
You say that the drive for the motor must be turned off as well. Are you saying the Gecko and/or UC100 need to be powered down as well? That would certainly defeat the purpose of having it switched automatically.