Why thanks Osiris II!
Would you please show me the Ancient Egyptian text that says Ay, had to marry Ankhesenamun. i don't mean some Egyptianologist text proclaiming that rule, but an Ancient Egyptian Text. Fact is, we don't know exactly what the mechanics of succession was. Once we believed in the heiress prinicess theory, but today that belief is... on its way out the door. Except for the successions of Ay, and Horemheb. Please see Women in Ancient Egypt by Gay Robins pages 26, 27, 150-151. A co-regency with the reigning widowed Queen would have given the very powerful Ay all the "rights" to the throne he needed. As Queen, her part in the co-regency would have been extremely small, as it should have been in Hatshepsut's day. Except Tuthmoses III was a very small child, and Ay was a very adult male. Again, to support your position, of "having to" i would appreciate those Ancient Egyptian sources.
i can understand your belief that a plague would be hard press to last 10 years in the Middle East. Except i read where King Mursilis II in the Plague Prayer Text, states the plague that killed his father Suppiluliumas I, and his oldest son lasted for at least 20 years in Hittite. A much cooler and hardier place than the Middle East.
Not all plagues take a great deal of time to kill. Because Mursilis II in his Plague Prayer Texts doesn't tell us the symptoms or lenght of illness of the plague. That was in Hittite for 20 years and brought into Hittite by Egyptians in Northern Syria, we can't say what kind of plague it was. Bubonic plague for example has an incubation peroid of 2 weeks, then you died quickly. i can see where a healing priest, treating the King's son symptoms of bubonic plague. Might claim it was poison and the final appearing syptoms the buboes, were the poisons being defeated and attempting to leave the body. Until the victim's death, brought on by over all weakness of fighting off the poison not of disease.
Of course Ay or even Horemheb might have paid someone to murder the young man. Then again, he may have died of a tooth infection that spread to the brain, another very rapid killer. Or even appendicitis, yet, another rapid killer. We don't know, can't know, and all thoughts are just spectulation. However, as long as we don't have substancial evidence, it is best to stick to reasonable, theories not murder this, murder that and they all died so, X could....

Claiming, using a mantle of "expert", that we do know.