No because the runningback already has possession. Like I said the key is "with possession" a receiver that is falling during the reception is not "with possession" until he finishes falling, his feet are immaterial (unless one comes down out of bounds), what matters is possession, he didn't have it. An RB that already has possession ends play when the ball touches the line so there can't be a fumble because play is over, play isn't over on a pass until it's a reception or not.
I believe you. If Dallas had been on defense it would have been ruled a TD.