Bernard and Isana have no secrets. Bernard knows who Tavi is. That's pretty much stated in FoC. I don't have the book in front of me at the moment, but there is a part when Isana is talking to Bernard and says something like "It's happening!" or something like that. And Isana also asks Amara if "she had come for Tavi?".  He would know exactly how old Tavi really is and he probably even knows about Isana's "tampering".

But you are right, it doesn't exactly add up by a year. Maybe it's just a slip, although Jim doesn't slip on the details.

I am not so sure about Bernard knowing.  In CuF he has no clue who Fade is.  My guess is that Isana told Bernard that Tavi's Father was a high ranking citizen and his family might come looking for him some day.  Given how few rights non citizens seem to have that would probably be enough of a story to get Bernard to be on guard.  If she had told him the truth then I see no reason for her to Keep Araris a secret from Bernard.