More good thoughts. I think you have the doomsday scenario covered there, which is nice. Cool idea.
Ran across this post: Best way to e-stop spindle and VFD?
Apparently the Onefinity controller hardware estop button just cuts power rather than putting the controller in the Builbotics estop mode. So I´d like to wire a separate hardware Estop button to pin 23 you mentioned, which, according to that post, DOES put the controller in Buildbotics estop mode. Then that would also stop the spindle through the modbus communication error to the VFD. There seemed to be a couple other benefits to the Builbotics estop mode over just cutting power. This seems doable.
What would extra bonus - it looks like the Hitachi VFD has an internal relay set to output an alarm signal on trip or power loss. If that could also be wired to the same pin 23 then when VFD/spindle sense problems that would in theory also shut down the controller (Buildbotics estop mode.)
A little confusing, but I’m going to see if any of the guys on that post can help. This sounds good to someone who doesn’t know what they’re doing (me), but not sure if it really works that way!