Can we use some common sense. Take the fumble out of the equation. On a typical run play, when there is an IW, Team A has the option to replay the down, or take the result of the play (where the ball was when the IW made the ball dead). Team A will evaluate the down and distance scenarios and make the decision that makes sense. The fumble in this Rule Question is basically the same. So, take the same question, and instead of 4th and 5, it is 4th and 15. The runner gains 13 yards when the fumble happens and then the IW occurs. Are you seriously going to say that Team A has the option to (1) replay 4th and 15 or (2) have it be 4th and 2 from the spot of the fumble.
That just does not make sense. How you you going to explain to the Coach for Team B that the Play didn't happen but Team A just advanced the ball 13 yards? Don't over read the rule.
So, the real result in this scenario, is that Team A has only 1 real option - to replay the down. Taking the ball where possession was lost and accepting the play -- leaves them short of the line to gain and results in Team B ball.