If they share a name with someone else then yes they forget the name completely.

As for canon mates talking about canon mates, it's all the same. If your character tries to say another canon mate's real name then they forget it. The true name can not be communicated in anyway until that person get their real name back.

As far as memories, with the example of Mokona if she remembers what they are and Fay does not, then she can try to explain to him but it's all really if he will believe her or not. He won't completely understand everything though until that memory is given back to him.