If all the plugs are the same colour unlikely to be an injector fault.
They, ie cossie engines don't need to be this rich, mine idled 100% perfect at 0.97% CO, 137ppm and NO CLOSED LOOP, oh with 200,973 miles on it, on GREYS.
Best thing I ever did was ditch the w/m ecu, wouldn't even use a free one if already set up.
All you can do is go through the usual, which you certainly seem to have covered.
tabetha