[Jim S]

If A fumbles this becomes a loose ball play. If A accepts enforcement from the previous spot. 15 yards to the B25 1 & 10.

If the receiver is downed before the contact you have a dead ball foul. March 15 from the subsequent spot. 1 & 10 from the B15. If the contact is before the receiver is downed, you probably have a roughing the passer call against B. 15 yards from the end of the play. 1 & 10 from the B15.
If you don't think it was roughing the passer then you still have a D foul and the enforcement spot is the basic spot, in this case the B 30, and you STILL have 1 & 10 @ the B15.

As far as "was it flagrant", was it flagrant? What I'm looking for is did the player intend to inflict punishment? Could he have stopped? Did he come all the way in with his head down or lower it at the last to make that contact?