If he had ran down the field to the other end zone I think he would have scored two points not the six scored with a touchdown. If he was stopped the play is blown dead and is a failed extra point. This rule was changed recently, maybe at the beginning of this season. If I am wrong I am sure one of the experts will correct me.
That is correct. And after Roby fumbled on the run back, NE could have picked up the fumble and ran it back for their 2 point play. So it could have been INT, fumble, 2 point for NE!