How does it affect an NPC trying to remember? If he fails a saving throw will he get another one? If so, when?

You say "feat that exists". Are we talking Core, 1st party, 3rd party? The feats you have linked are all 3rd party but I'm not sure whether you are aware of that fact.

RAW I fully agree. RAI, I have my doubts.

Hmm... I wasn't aware of scalable items before! This opens a completely new path to explore.

I have missed the fact that Bestow Grace should also be CL 18, so many thanks.

It is going to be a high intrigue campaign, with emphasis on social skills as well as fighting. Furthermore, the campaign will be 1-20 (levels), 1-10 (mythic tiers). From experience with these players, one of them will be full on Cha, the other will probably mix it up with Int for skill-monkeying. The last one... Well, she always plays something different so I don't know. In regards to your question, it is safe to say they will all have, at one point, more than 20 Cha, with at least one of them having 30 Cha or more.