From a set of resistance measurements it seemed that there was a short circuit at button D (Time Cook). This is quite possible because there are two bridges where Conductor 5 (energizing, or incoming) crosses over Conductor 8 (detecting, or outgoing). I'll call these places 'bridges'. If this was true, then cutting the energizing conductor would de-energize the outgoing trace that is wrongly giving the signal. 

I therefore cut the trace at the spot marked by the blue circle. The problem disappeared.

Unfortunately it all the buttons on Conductor 5 is now disabled. This includes the original ghosting button "Time Cook", and "Power Level" and "Stop Reset". This basically means that the microwave oven cannot be used for programmed functions any longer. But the "Min +" button still works fine, so I can still heat things at full power for a minute or longer. I can stop the cooking by opening the door, and I can restart it by pressing the "Start" button, so there is a measure of control. 

But this is a hack, and I'm still aiming for a repair.