That's hilarious.
That narrows the time table even more than I did anyway. Now, Gwen has to basically sleep with Norman within 12 hours of that rescue, because her father exonerated Peter the next day. So, daddy's girl left her aged father immediately after that trauma . . . oh forget it.
Even accepting all that (which I don't), it only takes care of "why (and when) would Gwen sleep with Norman?" It doesn't fix the fact that Sins Past turned Gwen into a bad person. Seven months she doesn't tell Peter, pressuring him to get married the whole time. She moved to London because Peter didn't propose immediately after her father's death for crying out loud.
You can forgive the infidelity on the grounds of human weakness. You can't forgive seven months of deception on the grounds of human weakness. Fear is often - if not usually - an element in immoral behavior. That she lacked the courage for seven months to come clean (again,
while pressuring Peter to marry her . . . and expecting him to take on the responsibility of raising her kids) - or failing that, breaking it off - is ongoing immoral behavior. She was scared? Too bad.
It needs to be retconned for the character to be salvaged.