2-13 only defines forces STARTING in the field of play.
2-13 says NOTHING about where the force BEGINS, which is why this whole discussion is taking place.
2-13 also says:
Responsibility for forcing the ball from the field of play across a
goal line is attributed to the player who carries, snaps, passes, fumbles or kicks
the ball, unless a new force is applied to either a backward pass, kick or fumble
that has been grounded.
So if a player in the end zone loses the ball out of the end zone by a fumble, and with no new force, that fumble is the cause of the ball going from the field of play (back) into the end zone, then that is the force that last put the ball in the end zone.
You say B is allowed ot end the play in the end zone. That's true, assuming he never leaves the end zone. Say B intercepts a pass in the end zone. He runs out to the B5, fumbles, and the ball rolls back into the end zone where his teammate falls on it. Touchback? Of course not, it's a safety, because B caused the ball to leave the end zone, and then returned the ball into the end zone.
This play is in essence the same thing. B legally had the ball in his own end zone, he was responsible for the ball leaving the end zone, and then coming back into it. He was the one that applied the force that caused the ball from the field of play into the end zone, which meets the requirement of 2-13-1.
Nothing in 2-13-1 says the frce has to START in the field of play, only that we only consider it when the ball goes from the field of play into the end zone. That's exactly what we are considering here. There is nothing in 2-13-1 or 2-12-2 that says this should be a touchback. To the contrary, they describe why this is a safety.