Both shorting wires go ultimately to the live side of the points. If one set of points is open, then it would seem to me that the entire circuit would be open, even when the key is in the closed, off position. Maybe what the problem is, is that one or both sets of points are considerably out of adjustment, so that when one set opens to make spark, the other side, which should be closed, is actually still open, leaving the whole circuit open. If not totally misadjusted, then maybe the mag plate has so much slop on the center bushing that is effectively causing the same problem of misadjustment.

A lot of the older, smaller Mercurys had a stop button with a third, grounding terminal in them. If you tried to use a regular switch, they wouldn’t shut off. I always felt like that was because the whole dwell strategy on their magnetos was such that there wasn’t sufficient overlap of one set of points being grounded when one was open. Nobody ever told me that; I just assumed it….God help me if I am wrong on this forum….