Altering the initial mechanical mixture adjustment will have no significant effect unless it's altered beyond the ability of the lamda system to maintain stoichiometry, which will probably knock all the gases beyond the cutpoints.

The 0.1% O2 reading indicates the catalyst could do a little more if it was hotter, but your HC and CO are good. The biggest problem on these engines is usually meeting the 15 MPH HC cutpoint when the catalyst is coolest. That's why the engine should be as hot as possible when you start the test.

NOx generation is a function of peak combustion temperature. (It has absolutely nothing to do with coolant temperature.) Retarding timing will reduce peak combustion temperature. Initial timing is fixed and not adjustable, but you can replace the short vacuum advance line rubber hose from the manifold to the nylon tube that routes to the EZL module with another piece of rubber hose plugged with a slug of silicone sealer, and/or you can replace the R16/1 resistor with a shorting plug that you can make out of a piece of wire. This will reduce spark advance at the speed/load conditions of the test and dramatically reduce NOx. It will also increase EGT, which will heat up the catalyst and allow it to promote more reactions, likely consuming the last remaining fraction of O2.

The above is thoroughly discussed in other threads in the last couple of years. Search under my screen name.

And please post your final test results.

Duke