Changing the drive is no help, it still needs a 10V speed reference.
I'd like to know, what's wrong with the high voltage digital potentiometer that I suggested? That will work and it is by far the simplest solution. Is the problem that you cannot handle the surface mount aspect of the part?
Abandoning the digital pot idea and doing this with op amps is easy so long as you have a 12 to 15 volt power supply. A simple LM358 with a gain of two and a low pass filter (one resistor and one capacitor) should suffice to convert the PWM to an analog output of 10 volts.
You'd only need about 50 ma of current. Using the drives +/-10 volt reference as the supply requires a high voltage rail-to-rail op amp and those don't come easy or cheap as I pointed out in post #8.
PS: Isolation is not a factor here, it's not required. MarkT's original comment was about the drive's lack of a PWM input which is the Arduino native analog output method. This is not a surprise in the motor drive world as +/-10 volts has been the standard for the speed command since drives became solid-state devices.