I just received a reply to this question from Shaw. Someone else also sent him an inquiry. I suspect it was someone who was following this thread. I will copy and paste the whole reply. It includes the plays someone else sent as well as mine.
"Jason –
I recently received a similar request below – Let’s look at it 1st:
A request for ruling on the scrimmage down following would be helpful to many of us who are not certain as to the enforcement spot for the Roughing the Passer penalty, when the ‘fourth down fumble rule’ must also be considered:
End A80 catches (by full definition) a legal forward pass at the A-42 and advances to the A-44 where he fumbles the ball. A70 recovers the ball at the 50. B99 was flagged for roughing the passer.
Ruling: The roughing the passer penalty is added to the end of the last run when the run ends beyond the neutral zone and there is no change of possession during the down (Rule 9-1-9-a&b). In your play the 4th down fumble rule is in effect so A80’s fumble at the A-44 is recovered beyond the spot of the fumble at the fifty by A70, so there is a forward fumble and no change of possession during the down. The ball will be returned to the spot of the fumble at the A-44 (Rule 7-2-2-a, Exception 2). Enforce a 15-yard penalty from the A-44, and it will be 1/10 @ B-41.
The play you describe:
4th and 10 at the B-30. A10 takes the snap and throws a legal forward pass that is caught by A80 on the B-5. He is hit and fumbles the ball forward where it is recovered by A20 while grounded at the B-1. A flag was thrown for roughing the passer. Ruling?
The penalty will be enforced from the A5, ½ the distance to the goal and an automatic 1st down.
Same play except the fumble is recovered in the end zone for an apparent TD. Ruling?
Same as above, the penalty will be enforced from the A5, ½ the distance to the goal and an automatic 1st down.
If the forward fumble went into the end zone and was recovered by a Team B player, then the enforcement would be at the previous spot because there was a change in possession during the down."