He also trapped the off arm when he executed. I think that's why the other guy's arm really got hurt - he didn't have any leverage to try and pull out, and since he was incapable of tapping the ref may have come in a little late.
I wasn't 100% sure what you mean by this and went back and watched again. It still looks like a classic armbar from guard to me. It's not unusual to have both arms in, otherwise you'd probably go for a triangle. Often when one arm is free, the person getting armbarred has cleared one arm but not the one caught. Mike got totally caught cos he was had just escaped (bad-ly), rolled into guard off balance and clearly wasn't anticipating submissions.