1) Maintain the "you're not sure about anything right now" stance.
2) Don't let her return home.
NO MATTER what she says... the FB is what caused all the problems. If you wants back in she must be prepared to give it up completely. DO NOT COMMUNICATE THIS YET. She needs time to process her disappointment and mourn the loss of her dreams. Let her stay at FIL or D21. You will not lose her because she is already lost.
If she says, "But I told you that there is no one else." ... Your reply, "Yes, I heard you, but my trust in you is gone. I don't believe you anymore. I need space and time to process my thoughts. If you cannot give me that then I know that you still have complete disregard for me and I have my answer."
I hope you can do the above... and I'd like to hear what other people think of that. I will try track down Sandi2 and ask her to have a look into your thread. She had an EA... and she would be able to explain things better to you about how the above works. This is a link to all of her posts.