Well I certainly wouldnt want to start a debate on that particular subject in the *traditional catholic* section of all places. But Im hoping to find an authoritive answer on whether the title St. Jonas is, on its own, a rock-solid infallible argument in favor of the historicity of the book of Jonah.
I don’t think you are going to find anything as rock-sold as you desire. There is, as yet, no solemnly defined, de fide teaching on these matters. However, I think you can infer certain things from (1) the words of our Lord, (2) the witness of the Fathers, and (3) the liturgy of the Church (which includes the martyrology).
(1) Our Lord seems to refer to Jonas as a real person:
“Just as Jonah was in the belly of the whale three days and three nights, so will the Son of Man be in the heart of the earth three days and three nights.” (Mt. 12:40)
“An evil and unfaithful generation seeks a sign, but no sign will be given it except the sign of Jonah.” (Mt. 16:4)
“Holy Mother Church has firmly and with absolute constancy held, and continues to hold, that the four Gospels…, whose historical character the Church unhesitatingly asserts, faithfully hand on what Jesus Christ, while living among men, really did and taught…” (Dei Verbim, #19).
(2) The opinion that Jonas is anything other than a real person is unknown among the Fathers; Sts. Irenaeus, Cyril of Jerusalem, Augustine and Gregory comment on Jonas as if his existence should be taken for granted, and St. Jerome warns, “I am aware that some will be incredulous that a man should be preserved three days and three nights in the belly of a whale, to which the shipwreck had led him; these people are either believers or non-believers: if they are believers, they are obliged to believe much greater things.”
(3) While the canonization process that is usually regarded as infallible was not used in declaring Jonas or any other Biblical figure a saint, the liturgy is considered a component of the living Tradition (CCC 1124). When Holy Mother Church celebrates the feast of particular saints, She is affirming their existence and holiness. By including Jonas in the Roman Martyology, Holy Mother Church is saying that you can safely pray to him to intercede for you to God.
Hence, you are clearly on solid ground if you choose to believe in Jonas. Perhaps the burden of proof should then be upon those who wish to deny his existence and the historicity of the book which bears his name. Remember, Bultmann doesn’t have the title “Doctor of the Church.” He’s not canonized. He wasn’t even Catholic.