Merlin (if you are out there) I assume these changes I had to make are all do to the VGSoff of the fet being incorrect?
Yes almost certainly. The J201 needs less than -1.5V to turn off- very sensitive!
However, I am surprised that you needed to reduce
the threshold resistor, because that would increase
the gain of side chain, yet that FET needs less
voltage to drive than my version! Did you use a scope to bias the FET, or ears?
EDIT: Ack! Just noticed that D1 and D2 are both backwards in my schematics! I am updating them now. This would explain your problem, sorry!