Unable to start - 12V battery OK

If that's all the trim you've got, then you are missing a lot. No bare metal should be visible.

 

Stop making voltage measurements at the battery itself. measure at the jump posts under the hood, because if its not there thats your problem. Not sure if the G3 is like the G2 which has 2 fuses in the positive assy thats bolted to the top of the battery positive post.And the G2 has unpluggable leads off that assy so you can disconnect those instead of the sparky spark unbolting the ground strap.
.Remove the negative ground strap off the chassis and sand down the area it mates to. Remove the 2 battery terminal connections and sand down the posts. Clean it all off with choke cleaner. Apply synthetic lube to all connections and bolt back up. More grease the better.
Then when done measure at the jump posts.