Personally I am somewhat cynical about the supposed correspondance between an
Eygptian queen and the Hittite king, which I understand is known only from records
about a generation after the fact. While I do not doubt that Supp. tried to take
advantage of the succession crisis in Egypt, his complaint about the death of
Zananza proves as much, I wonder if he really required an invitation to do so. Or
that Ankhesenamun would be fool enough to chose the son of a barbarian monarch
over what must have been a plethora of Egyptian possibilities.

Egyptian precedent, Sobekneferu for example, justified female succession in the
absence of male heirs. Possibly Ankhes chose to ally herself with her probable
grandfather, (and great uncle) as co-regent. He was probably the most powerful man
at the Egyptian court, the one she trusted most and had the further advantage
of being much older and so likely to leave her to enjoy an independent reign
once she was more secure and experienced. However as it happened she died too -
or so we must assume - leaving Ay sole pharaoh.

This reconstruction may be contradicted by Tut's funerary reliefs which show
Pharaoh Ay presiding over his rites with no trace of a co-regent, suggesting that
he directly succeeded his great-nephew (?).

Personally I rather like the idea of Merytaten/Neferneferuaten as the queen of
the Hittite letters but that doesn't seem very likely. More likely the letters
were a propaganda invention after the fact. The Hittite kings would have had
excellent reason for wanting to cast doubt on the legitimacy of the Ramesside
dynasty.