The Woodward book and the anonymous NYT memo have made it clear that if Trump gave the order they'd just ignore it and placate him until he became distracted with something else. If he gave an unlawful order, they'd be obligated to ignore it. Despite his plays at being a dictator he's the least respected man to sit behind the desk in the last 100 years.
Yes. I used to worry about this, but not any more. It's become clear that everyone around Trump spends more time babysitting him, and stonewalling him, than they do enabling him. There's no way a nuke order would be obeyed. The "adults in the room" wouldn't let it happen. I suspect very little that Trump orders gets done without his babysitters' approval.