Yep.. i said it might answered some mystery because Yuu was basically the only one we know that stayed at the Hyakuya's laboratory where all the experiments about Four horesman of John, SOTE and the demon possession took place. He had been there for 5 months. Unlike others who are experimented in the orphanage. So I'm pretty much sure Yuu at east saw something about the experiment that the Hyakuya doing. And like you said, they surely need to erased and altered his memories since they don't want the experiment to be known and also for lessening the trauma. But what happened is Yuu forgetting everything even his own name and parents right when he left the laboratory and was brought by Saito.
Even Mika and the others remeber their own family name. But Yuu forget it all. Also when I said contradicts is when Yuy said " his parents were used to be gentle to him " and then he said " his parents never love and kind to him "
The hyakuya might even give him a false information in exhange of the lost memory that he had