Does the phrase "a sinner exposed to a threat of punishment" mean that the penalty for someone who believes a transgressor and accuses an innocent person is equal to the penalty for the transgressor himself, especially if imprisonment or death results from the accusation, and what is the punishment for such individuals, and what is incumbent upon them?

The sin incurred by believing a transgressor is not equivalent to the sin of the transgressor himself. Each has his own distinct sin. Allah is just; He wrongs no one and recompenses every person according to what they have earned. You should occupy yourself with obedience to Allah, and remember that Allah holds accountable even for an atom's weight of good or evil, as Allah Almighty says: "So whoever does an atom's weight of good will see it, And whoever does an atom's weight of evil will see it."