912iS not cranking

I remember (I think) that the RV-12 has a spar pin warning system that will defeat the starter relay.  Magnetic switches on the spar pins I believe.  That would not explain why rotating the prop would fix it, but maybe that is a red herring.  Possibly just movement of the aircraft while rotating the prop would make the faulty spar pin switch engage.  

Rv12 Spar Pin Light - VAF Forums (vansairforce.net)

Another good idea; but the spar pin fault light was not illuminated, and I did try engaging the spar pin override button to no effect.

although it would be a good troubleshooting area as the symptoms appear similar.