Unnecessary Roughness

[Scooby]

I have an enforcement question.

1st and 10 from the 50. A45 breaks away and is on the 5 when, A30 hits B10 at the 40 yard line. The covering official judges the hit unnecessary and throw his flag. A45 continues running and scores a touchdown.

What is the enforcement?
I believe that, if B accepts the penalty, the score is negated, and the 15 yard penalty is assessed from the 40 yard line resulting in 1st and 15 from the 45 and the clock starting on the snap.
Is this correct? And any references would be appreciated.
I work the National Federation Rule set, but would be interested in any other rule sets as well.
Thank you