^Good lord, we're not talking about a regular plug and play fusion device here! It's COLD fusion
technology. It's a whole different ball game!
I think the point of the entire scene was to show that these were still kids playing with adult things.
So the kids get a pass because Starfleet had the poor judgement to give them the flagship to play with in the first place. Incompetency at the highest levels of SF Command? Well, I guess that does follow tradition. So, whereas in the past Our Heroes™ were the smart ones and the Admirals were stir crazy, now the whole lot are idiots.
Uh, no. No one said incompetent. Green? Yes. Reckless? Yes. A little too fearless? Yes. And no one gets a pass, either. He undoubtedly embarrassed Pike over the year. And, Kirk was finally punished, and he should have been. Not so much for the PD, but for his behavior and lying in his official ship's log (a federal offense in the U.S. in today's world).
We never saw the young Kirk Prime. We don't know how his judgement evolved. Twice in his older life he was critical of himself. He said once he relied too much on luck thinking it was skill, and another time he said he was the "fools rush in" type.
As far as the volcano goes, a more mature Kirk or Kirk Prime would've saved that world, too, but his methods would've been more thought through and "conventional," I'm sure.