I've finished up through Chapter 5 of Famicom Detectives Club (I guess I should call it The Missing Heir since there's two games). Just a couple of theories so far: The guy who "found" the MC has to be involved in all this somehow. He might be this Kazuto Ayashiro guy, but he's probably at least the person the MC was supposed to meet at that cliff. The game has made a point of mentioning that no one ever goes there, so the fact that he just so happened to pass by and find the MC seems too coincidental.
It's also strange that the game made a point of mentioning that the MC is borrowing a shirt from the guy who found him because the shirt he was wearing at the time of his accident got "dirty." There has to be some sort of incriminating evidence on or in that shirt, but I guess we'll see!
Yeah I've thought he was the culprit the entire time, though it does seem perhaps too obvious.