From a Doylist perspective, fourteen books in, what purpose is served by having Mab be wrong about how Maeve was infected? All the vectors are dead, save Lea.
Mab thinks the problem is solved because the source of the infection has been found, the Knife. But if she is wrong, the real source, Elaine is still alive and potentially causing trouble.
Okay, fasten down your tin foil hats ladies and gents, it is going to be a bumpy ride...
Using your Doylist perspective, the books are about balance, Summer Court verses Winter Court etc.. I remember a word of Jim from long ago that suggested the possibility of two star children, Justin adopted Harry and Elaine the theory goes is to gain two star children to use for his own purpose, i.e. enforcers. Or he thought for his own purpose, but there are hints that he was betrayed.. What if Justin was merely a cat's paw? He was used to raise up and trained these two
powerful wizards possibly star children to be enforcers. However to balance the forces i.e. the Courts, the White Council etc, only one star child is needed... So that fateful day when Harry came home early from school the master plan was about to be set into motion.. Elaine, successfully enthralled, malleable for the plan.. Harry resists enthralling, not useful for the master plan.. Harry always believed that Justin had sent HWB to kill him, but after his reliving it in Ghost Story he is no longer so sure that this was the case.. So why did he survive? We'll get back to that in a moment..
He does survive, blows up HWB, goes to his godmother, who happens to be a powerful noble of Winter, gets some confidence goes back to save Elaine, kills Justin in a fair fight.. Elaine says she
freaked and ran off to the Summer Court for protection.. Was she still enthralled? What was drilled into her by Justin while Harry was off getting juiced up by Lea? Elaine carries the infection to Aurora etc, etc, etc..... Balance is all screwed up, or that was the plan..
It failed because Harry survived, he is the star child who has power over Outsiders, because his mom fell in love with Malcolm, it gives him the power to influence Elaine so she helped him to get to Aurora when all she wanted to do was run and the consequences be damned....