I don't think it's foolish. It's not like he hasn't given you good reason to think it.

[ And if it is, then Yuuri's a fool too. Foolish for being frightened on rainy nights, tightly wound and anxious on Friday mornings and sick with fright whenever there's a moment she doesn't know exactly where Ruu is. ]

But... for what it's worth, I don't think he could pull it off. He might be able to fake being Ardyn Lucis Caelum, but he doesn't know a thing about Ardyn Tenmyouji.