I'd like to get Fall from grace to join my party but the option to talk to her ten students isn't coming up for me...She just says ' you'd better come up with a better reason than that for me to join you' and the conversation ends.

Nevermind! it turns out that you have to exhaust all conversation options with her beforehand

I tried all the ways of asking, but no combination worked until I went back to the earlier questions and said I needed her help, then told her I had lost my memories, etc. -- a full-on confidence in her. I also had cast the "Friends" spell before the conversation; not sure if that was necessary.

Indeed, you specifically need to ask her for help, and then tell her how you woke up in the Mortuary with no memories because you apparently were dead. Once you do this, when you're trying to get her to join, there'll be an option to ask if she is "interested in traveling with an immortal amnesiac who is searching the Planes for himself". This is where she gives the quest to talk to the girls in the brothel. Also, there are Charisma checks in that dialogue (15+, I think), but those seem to be cosmetic, that is, they affect your manner of speech but not the outcome.