One shot is not enough to stop the need for deadly force. I bet he could have fired 3-4 rounds into each teen and if he had left it at that, there would be no trial at all unless the family just wanted to push it.
Only when the targets were nearly unconscious would they pose no threat.
Threat was there when they broke into the home unannounced. There is no room for debate there. 2 teens against one senior adult in a community that has had a rash of break ins including his home.
He had every right to fire more than once at each target. 3 at each target at a minimum I would bet is ok.