While I'm kinda into theory that Shuichi knew/suspected what Kaede was planning I was rewatching the 1st trial and there is a moment where he laments (in his mind) how he should have noticed what she was doing but didn't which unfortunately does kinda disprove that theory. I think.
He does, and I admit that - setting Tenko’s later commentary of Shuichi ‘going astray’/’deluding himself’(JP) aside - Shuichi deluding himself to the point of, say, creating a whole persona in his head for the sake of continuing the killing game... does sound a bit strange. But the logic here is less about the idea that he ‘doesn’t think his lamentations at all’, and more about what he could do under scrutiny- reinforce to himself that he didn’t know something, whenever he feels guilt. The lack of ‘direct action’ he took is something he can easily brush over to himself as ‘incompetence’ as a way to protect himself- I think one time to get into gear regarding this psychologically, was when the spotlight was on him and he went silent for a while, some time before the protagonist switch.
That said, it’s not something that’s provable and I’d say depends on how much you (general you) believe in the Shuichi we know. Personally I feel as though the days we have spent out of his head since meeting him, is significant enough to be missing a great deal of who he is, and it’s partly for that reason I doubt him for what he’s really capable of, even to himself.