I would think all of what you are saying is correct.
Doing real tchuva will benefit. However, it won't erase the punishment of the past.
Benefit? If we go by what Rabbi Mizrachi says, she signed up to go to heaven. She'd get all of what she would have got after in this world. Better to fight against it to make it easy on yourself. I guess she can still become a Noachide and not marry the guy, and doing teshuva for that counts for something, but no one claimed you don't get what you deserve, you just don't want to scare anyone with it (or do you?).