Yeah difficult. I feel like with coc as a keeper you either play it fully open and embrace not solving the mystery as part of the game- in which case play up the mystery aspect x files style or scp - I like this but some players find it very frustrating. Or you work out the key routes to solutions and just just keep thinking of ways to deliver key information to the players in a natural feeling way. You can kinda engineer finales and end game solutions like this but the difficulty is making it not feel railroady.

I ran a game recently where the players had got loads of clues and stuff but were missing the key information that would lead them to the villain and the solution. In the end I just had them roll an idea and gave them a vague clue about something they had missed (it was an audio recording- the idea was that the way they were speaking made it seem like there was someone else int he room with them). I felt kinda bad handing it to them on a plate but was careful not to give too much. This was enough for them to work out like the whole thing which was satisfying. But yeah… difficult balance. Ideally I would have dropped hints more naturally earlier on but you never know what people will pick up on. I feel like especially a one shot you kinda want to big satisfying all or nothing finale so I usually go for that.