Can you just manually copy and paste the www folder from the web control version you need?
I've got a workbee and ran into some trouble with the web control after updating using the Ooznest firmware. I ended up just updating to the latest duet firmware, web server and web control and have been sweet ever since.
The control is quite fine. Increasing or decreasing rpm by 10 with M3 shows the change. In practice there is much more error than that during cutting. More like +- 50 rpm. The torque/phase current control is nice. I can physically jam the spindle and the current through the windings will simply be whatever I set as maximum motor current. I imagine that abuse would fry the included ESC in seconds.
I recently put together a workbee CNC with a duet and a brushless spindle paired with a vesc ESC. This thread came up when I searched for vesc and duet spindle control.
I found the vesc "servo" input is really a regular pwm input. It is not particularly picky about pwm frequency either. So hooking up a duet pwm output to the vesc servo input works fine.
It works pretty well and the vesc has its own PID loop to hold speed steady.