When I was taught the lean to clean technique it was, advance power to say 50% (about 2000 RPM in a fixed pitch or with the props full forward) and lean for peak. This is nowhere near going to wide open throttle and full RPM!
On my engine 2300 rpm and 23 MP leaned to best power is 65%. General wisdom is that below 65% you can set any mixture you like and not damage the engine. Obviously no cooling airflow will mean operating at this level can overheat the cylinders on the ground, but for the purpose of cleaning plugs 20 seconds isn't going to heat the engine too much (although watch the gauges!).
On a multi channel analyzer when you switch to one mag there should be a pretty uniform rise in EGTs across all cylinders and then a return to the starting values when you go back to both.
A duff plug will show a plunge in EGT, No change in EGT may imply the mag ground switch is duff.