Inadvertent whistle in the end zone question--

[Bullycon]

Case play 4.2.3-D does not mention the momentum exception, only that B "circles back into his end zone." The phrasing would lead me to believe it was not momentum, but his own choice that took him into the endzone.

Thus, B was responsible for the ball being in the end zone. Ball is dead in the end zone. Result: Safety.

In your case, Rule 8-5-3-d is all you need. "It is a touchback when: A forward pass is intercepted in B's end zone and becomes dead there in B's possession."

You got it right.