When we hadn't heard from you for ages I was worrying that difficult child had reverted to type. Robby, you and husband have done everything you could have, and more. As you said, you have been lied to, more than you realised. He's going to be back, when he wears his welcome out by not following the rules at his brother's. But in the meantime you and husband need to heal.
If difficult child wants to live with girlfriend, he has a lot to learn. They need income, they need somewhere to live and to make it work they need to take responsibility for their own bills. He knows this and demonstrated while in the Marines that he CAN do it. And from here - you mustn't rescue him any more. Nobody should. Eventually those who want to rescue him will be worn out and fed up. You can't keep using people forever, eventually you wear out your welcome. Sooner rather than later, with some people.
Marg