All valid questions no doubt but in my opinion you are overthinking it a bit. K2 is mostly used Joomla extension today and just because they did not release new version does not mean it is dead, to me it shows stability. Last version out in Dec 2014 and it still works with all Joomla versions in between which we cannot say about majority of J extensions .
As far as activity , I see movement here , http://prntscr.com/a6ldo2
and looks like we will have 2.7 soon.
Look at YJSG , we released it in July 2010 and since than we moved only to 2.2. It does not mean that we dont work , it means that we found a base that worked for us for the past 6 years.
So I say , use K2 and dont be scared. If anything happens in between we would also update the whole template arsenal and find suitable solution but knock on wood , we did not have to do this so far. As for developer promise to release new version and not honoring it , well , developers usually never meet the deadlines