Rule 7-3-7-c Exception says that if the penalty for an illegal forward pass is declined, the ball belongs to team A at the previous spot. Thus, the ball is not dead in the team B end zone with team B being responsible, and no safety.
Can we get technical here and say that the offended team didn't decline, rule 8-3-4 did

Then the ball belongs to team B in their EZ and they are responsible for it being there.
This exception would make a lot more sense to me if it only applied to team A illegal passes. It is very, very odd that after a change of possession and a declined penalty, we would award the ball to team A at the previous spot. I can't think of any other rule that has a result this strange. Also, what's the next down after team A declines the penalty (assuming from a scrimmage down, not a try). The continuity of downs is broken (5-1-4-a), and without an accepted penalty, that always results in the next down being first down or a free kick. I can't imagine that the next down is 1st!
Play: A's ball 3rd and goal on the B-4. QB A12 drops back and throws a legal forward pass into the EZ where it is intercepted by B20. B20 runs out to the B-1 and then reverses field back into the EZ. As B20 us about to be tackled in the EZ he throws a forward pass toward B23 who is at the B-2, the pass falls incomplete.
Ruling:
- If the penalty is accepted, then it is a safety, 1 points for A.
- If declined, A's ball 4th and goal on the B-4? Clock ready?