i have the same valve dripping problem.
it was dripping a little (gallon per day) and after putting new valve picture was the same, no change at all. 3 months later, after burner draught fan replacement it dripping increased to gallon per 2 hours.
so - there is almost no worn parts, new valve, but after shutting and starting hot water service dripping increased all of a sudden...
from what i saw on your site there is no definite solution about that and it does not look like problem with minerals in pipes or new valves or general water pressure - parts are the same, ONLY their condition could be different.
WHAT changes during shut/open water for boiler? probably expansion tank behavior - i believe it has to be handled somehow specifically - water released for example. because, when you put everything new in a new house you do not have such problems. and all devices have AIR inside.
i would greatly appreciate any comment on that subject