If he was pulling it back before hitting the half cocked position and let the hammer go, there wouldn't have been enough force the fire a round. If he was pulling it back beyond half cocked without pulling the trigger and let the hammer go, it would have stopped at half cocked. As you said, if he was trying to set the hammer down after cocking the gun, and he would had to have pulled the trigger.Not technically true. He could have been pulling the hammer back, or releasing it forward (yes, I realize that putting the hammer forward technically means pulling the trigger, but I'm giving him leeway knowing he knows jack shit about firearms) and accidentally released it. In theory it shouldn't have enough force to set the round off, but it's not impossible.
My honest guess is that he's overcome with guilt and trying to rationalize anyway it could be not his fault. The problem he's creating for himself is that he's coming up with elaborate (and obvious) lies and these lies are making him seem less like someone involved in an accident, and someone who's trying to hide the truth.