In your example, A does not score any points since it was A's force that caused the ball to be in the end zone. Who forced it to go out of the end zone is of no consequence.
Also, you say if B touches the ball it is dead. That is false. B possessing the ball causes it to be dead.
Here is how A can score one point on a non-kick try. A fumbles the ball. While it is grounded and nearly at rest, B muffs the ball into the end zone and falls on it.
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