Denford Triac Spindle Issue

[m_c]

The fact the drive runs with the SGR (Spindle Go Relay) relay removed, indicates either a fault with the sprint drive, or somebody has altered wiring.

I think I'd start with comparing the wiring to the wiring diagrams.

54 should go to terminal 49 on the connector board, and 55 to 47.
The enable circuit passes through the SGR, and the enclosure safety switch relay -

If the spindle still activates with the 51/53 wires disconnected from the drive, then there is an issue with the drive.

But that wouldn't explain why you're not getting the expected 0-10V output.
You are measuring the voltage between the 54/55 wires, and not to gnd?