I don’t know who Riley was either. We might never know.
Derek will go to any lengths to get his mission completed. Even if that means disobeying an order. But, he means well and loves his family.
Have you ever known Mom to shack up just some random guy? Marcus is one of my soldiers. But he’s different. He’s a cyborg that will live a very long time. He donated his heart to me when I was on the verge of death from a Terminator. I trust him. The end.