It came to pass that the power switch failed & the leash wouldn't turn on.  During teardown, it could be argued that the contacts were dirty. A complete switch rebuild revived it, but it's not the ideal switch for such a dirty environment.

The decision was made to embark on a software switch.  The idea is another hall effect sensor would detect a magnet in the latch.  This introduced the new problem of ensuring the latch didn't inadvertently move out of position.

After many test prints, a novel PLA spring evolved which would keep the latch either fully closed or fully opened. 

Getting the latch to align precisely required much removal of PLA. The big question is how long the friction point is going to last. It would be easier if it was in an area which wasn't handled, so it could be lubricated.

The difference between on & off has to be big enough to prevent accidents.  This is adjusted with the ADC threshold.

There can be no leash rebuild without a test showing it works.