My preferred way of doing a repair is to diagnose the problem with pin-point accuracy. This often means eliciting the undesired behaviour repeatedly while varying conditions.
In this case I need to make and break connections, and on a printed circuit that is not too easy. But I have a little bottle of silver conductive paint, and brought it out.
My hypothesis is that there is an unintended leakage of current between a probe line and a sense line in the matrix. This happens where the one passes over the other in a bridge.
So the next step was to identify the bridge(s) that gives the problem.
But first I had to undo my previous cut, which removed all power from the line. I will admit that I was trepidatious, because I did not know if my silver paint would be compatible with the printed conductor, or work at all. I had left the power on, and a few seconds after I'd painted the machine beeped and the fault condition was established again.
The matrix lines I suspect of involvement in the malfunction are Line 5 and Line 8. Line 5 is the probe line, and Line 8 is the sense line. They cross at two places, at bridges I call Arnhem and Nijmegen, shown on the picture below.
Cutting the track east of Arnhem bridge made the malfunction disappear, but also disabled the same number of buttons as did cutting the track west of Arnhem bridge.
This showed that the current leakage did not take place between Line 5 and Line 8 at Arnhem bridge.
Next I cut Nijmegen bridge itself, just before it crossed the first track. This removed the malfunction, and it only disabled Button H. To confirm that it was the crossing that cause the problem and not another leak somewhere else, I repaired the cut and made a new cut in Nijmegen bridge east of the tracks. This did not make the malfunction go away.
I made a few other cut/repair combinations, and I am now certain that the source of the leak is the crossing of Line 5 and Line 8 at Nijmegen bridge.
I can now start considering my options for repair. It might be as simple as nail varnish and silver conductive paint.
There is a danger that I might not complete this project, because the microwave oven is now usable, except for power level control. It reduces the urgency to complete the job.