Actually, having the possibility for somebody lying should probably be a pretty late-game thing, as it makes your belief network a lot more complicated, and I'm not sure whether this thing should display numerical probabilities at all. Instead of having to juggle the hypotheses of "Alice lied" and "Bob exaggerates things"

Perhaps the protagonist should start out being trained as an investigator on an island where the inhabitants have strong taboos against lying. Early cases are scenarios crafted for your benefit as a trainee. Eventually, you graduate to investigating real cases, and later on, you leave the island.

ETA: Possible alternative to using numerical probabilities. The player's findings are written in colored boxes, and the color of the box represents how likely the player thinks that individual belief is (ex. Blue=Practical certainty, Green=Very likely, Yellow=Somewhat likely, Orange= Unlikely, Red= Practically falsified.) Early on, players will be taught to make beliefs that are consequences of other beliefs at most as likely as the beliefs they follow from, and to assign lower likelihood to the overlap of multiple conditions, so for instance, if they have a box that follows as a consequence of three yellow boxes, they'll want to color that box orange.

I'd be happy to contribute writing to the project, if you're interested in having me.