How consistent this fiction is! It seems true

When do we say of a fiction that it is credible? Coleridge claimed that "poetic faith" involves a suspension of disbelief. If he was right, a literary fiction must be credible. But of course literary credibility doesn't mean truth. I think it means coherence. Fictional content must "make sense". If the plot is inconsistent, or a character is represented in a way that is inconsistent, something has gone wrong. It is coherence that makes a story credible.
That also applies to legal fictions, which are a classic subject of jurisprudence. I take them to be statements that are knowingly false but accepted in order to yield a good legal consequence.
Some legal and political fictions are used to provide a foundation for a whole system. Think of the State of Nature, or of the General Will of the citizens. These are fictional entities. It is knowingly false that the State of Nature existed as such, but it is accepted as an assumption leading to some desirable consequence. What consequence? The establishment of the sovereign (Hobbes), of civil and political liberties (Locke), of democracy and equality (Rousseau). These fictions serve as premises of an argument to provide a foundation for a legal or political system. I call them Foundational Fictions.
Other legal fictions are used to justify a legal consequence that couldn't be reached otherwise. I call them Justificatory Fictions. They are constituted by knowingly false assumptions accepted to justify a conclusion. One interesting example is the Roman Lex Cornelia: if a Roman soldier was captured and eventually died in captivity, his case had be treated as if he died when captured. Why that? A legal rule, valid at the time, deprived of the testamentary capacity the Romans who died in the hands of the enemy, but that consequence was felt to be unjust in the case of a soldier who made his will and died in captivity. So, because changing that rule was too difficult, Roman jurists used the Lex Cornelia fiction, taking the soldier as died when captured, in order to preserve the will's validity.
Other fictions in the legal domain, though not traditional, are what I call Cognitive Fictions. Consider the artificial reconstruction of the facts a trial is about, for instance by computer simulation. These fictions are not false statements accepted to produce a legal result. For they incorporate hypotheses on the relevant facts, and, when the hypotheses are correct, they are true. Hence these devices are fictions in a different sense: a "phenomenological" one, since their point is to give an artificial representation of the facts.
Finally, there are fictions consisting in entities that do not exist in the physical world but only in the legal one. For instance corporations. They do not exist per se: they belong to the legal world and, more in general, to the world of institutional reality (like money, taxes, financial crises, political boundaries, governments, etc.). How do they come into legal existence? By virtue of legal norms and acts constituting them. I call them Constitutive Fictions.
Now, is coherence a standard of fiction credibility? Consider foundational fictions: they need to support their conclusions with acceptable (though fictional) premises. For instance, a certain kind of story has to be told about the State of Nature, the way it was, the way we behaved in it, and so on. To be persuasive such stories cannot fail to be coherent.
Justificatory fictions follow the same logic. The story told in the Lex Cornelia fiction was false but coherent with the relevant facts and attitudes. Imagine pretending that the soldier did not die in captivity because, just before dying, a stork took him away and brought him to Rome. Such a story would be false, as the Lex Cornelia one, but surely less credible. Why so? Because less coherent with the relevant facts and background, namely human efforts and wars, not fancies and storks.
As to cognitive fictions, being made of technological representations they should be coherent in order to be credible. Constitutive fictions, instead, are not language entities capable of being coherent. Coherence doesn't apply to a corporation as such. However, these fictions are nothing at all without the statements creating them, and coherence applies to these statements. So, coherence provides a general account of fiction credibility.