No disrespect to Heiny Sensei of course, but I thought instructors weren't allowed to promote others to the same rank they hold (i.e., a 6th dan promoting another to 6th dan)? Just wondering...
If you are independent, who is going to "allow" or "disallow"? Rank really has to do with credibility and Mary's is very high. Mary's teacher was Hikitsuchi Sensei, now deceased. For quite a long time she was not promoted as she had no affiliation. Saotome Sensei arranged for her to get 6th Dan, which in his day was Shihan and was all the credibility you needed.
Things are a bit different now... 6th Dan isn't what it was; there are lots and lots of them. But folks should be aware that there are the "old Rokudans" and the "new Rokudans". I am Rokudan but Mary is so far my senior that the ranks don't mean anything... I trained with Mary when I was a Nidan; she was Godan then.
A teacher reputation is what gives their rank credibility. For most people, if Mary says someone is 6th Dan, that's good enough for them. Would their ranks have more credibility if some authority in Japan, someone who had never even seen these two teachers, rubber stamped the promotion? I personally do not think so.
In the old days, rank was never the issue. It was "who is your teacher"? And "how long have you trained"? We could add now, "how long have you been teaching"? Mary has been teaching for longer than I have been training. The students she promoted have been training for well over 30 years and have taught for over 20. I don't think that having some additional over-seeing issuing authority makes that any more meaningful.
Mary got her "authority" the old fashioned way... she earned it.