This is a point I have thought about before.
If everything else is a lie, there is no reason to believe that the birth date given is the actual date. We could be barking up the wrong tree by looking for evidence for events (Stanley Ann overseas travel, for example) that happened on or near his stated birthday. Or trying to explain that she would not be traveling with an infant just days after his birth. Some events would become much more plausible if he were just a week older than stated.
Remember a couple of years ago when he misspoke about his birthday coming up “next week” when it was really further away than that? Just another slip? Any one of these errors taken alone could be easily explained, but there are just too many of them which have piled up to make them all just explainable slips of the tongue.
That same thought has crossed my mind too.. but I usually sit on the sidelines to see how it all unfolds :p