I think the game is mostly fine but we're just bluffing that the villain is dumb enough to keep evidence on them?

First case is a wild logical leap where we assume what the evidence is exactly out of nowhere and then we bluff that the evidence is on the murderer when they should have discarded it or at the very minimum NOT bring it with them. We could assume the missing evidence was incriminating but guessing exactly what it is is a wild leap.

Second case for some reason the murderer didn't discard the most obvious piece of evidence once again, yet they thought to cover their tracks everywhere else.

3rd case I don't even think we got "decisive" evidence considering how high the bar is for proof. Any random excuse would have sufficed. Not to mention it took way too long for the main character to figure it out was murder instead of suicide.

4th case once again the evidence is on the murderer, not to mention we never even consider the possibility of multiple murderers and accomplices. If the murderer can slip cards into people's pockets without anybody noticing, discarding the evidence or even framing somebody else should have been child's play.

The final case it makes 0 sense for a "smart" detective to follow the directions of a news paper cut out when at that point he considered that person to even be a suspect in his brother's murder if he even thought the cutout came from that guy. No sane person is going to go to an isolated area of a potential murderer by himself.

The brother's murder makes 0 sense whatsoever. We never get an explanation for the knife being too light. Also if the brother knew he was going to die and who did it, it would make sense for him to point out the murderer at the very minimum.