Dear Ron
Excellent article! I would like to point out two things, though:
1. As the example of Switzerland shows bond yields can become quite substantially negative without a run for cash because, (1) there is a group of captive buyers like pension funds and insurance companies that have to keep on buying them for regulatory and ALM reasons, and (2) central banks and retail/commercial banks can refuse to print more physical cash or refuse to pay out your assets in physical cash. In Switzerland the SNB controls the growth of M0 very tightly and some commercial banks have in their T&C that under certain conditions they can refuse to pay you in hard cash.
2. Your analysis assumes a rather stable supply of sovereign bonds for the central banks. In theory fiscal easing could lead to a significant expansion of budget deficits and thus higher growth in sovereign bond supply. At least in some countries like Japan I would not put this into the realm of the impossible.