If the relay turns off the drive and allows the drawbar to move simultaneously, there will still be a 'danger zone' if the spindle was running a 7K and the drawbar was set to move. The adapter would release as soon as the VFD was disabled, but it would still be spinning at 7K.
The Tach idea would eliminate that possibility. Also, as mentioned previously, I don't see how you can disable the drive and then change the tools. What would keep the spindle stationary?
It occurs to me that there has to be some sort of home sensor on the spindle in order to change tools that have drive dogs, so unless I am missing something, why not just watch the home signal? i.e. WHILE <continuous home sensor signal>, do the tool change, WEND. The spindle would have to be stopped and any movement would halt the tool change . . . which would be a good thing, presumably.
I think this is the hot setup for the spindle, so that just leaves every other movement . . . LOL! Actually, I have magnetic sensors on the air cylinder, so it will be fairly simple to detect that the rod has reached the correct release and grab locations. If it fails to pick up a tool, it would move past the correct grab spot, and that should be pretty easy to detect also.