"And none of the above really engages the article which initiated this thread. What is the point of the book of Jonah? Is it the miracle of the fish, or is it the condemnation of the idea that God is a genocidal monster? Or is it something completely different?"
Actually, it does engage your original article. I suspect that you are viewing the book of Jonah from a historical-critical point of view. Thus, your questions and confusion about how it is relevant to the rest of the Bible. With a Christological interpretation there are numerous foreshadowings and parallels between Jonah and Jesus.
The book of Jonah (as do all of the OT books) looks forward to the day of Christ. God bestowed his unmerited favor on this people and sent his servant to preach a word of repentance.
The first thing to be established here is that the name Jonah in Hebrew means "dove" (See Strongs 3124). At Jesus baptism the Spirit of God is seen descending as a dove. The dove points to Jonah as sign. The dove was an alternative offering for Jews who could not afford a lamb. When John the Baptist saw the Holy Spirit descending on Jesus in the form of a dove it was a fulfilment of the Sign God had promised him - the sign of Jonah ! Jesus was the sacrifice for the common man. The poor man who could not afford the normal sacrifice.
"32Then John gave this testimony: "I saw the Spirit come down from heaven as a dove and remain on him. 33I would not have known him, except that the one who sent me to baptize with water told me, 'The man on whom you see the Spirit come down and remain is he who will baptize with the Holy Spirit.' 34I have seen and I testify that this is the Son of God." (BTW all three persons of the Godhead are present here, thus the Trinity is not "derived")
It is interesting that every time the Pharisees ask for a sign Jesus states that no sign will be given but the sign of Jonah.
38Then some of the Pharisees and teachers of the law said to him, "Teacher, we want to see a miraculous sign from you."
39He answered, "A wicked and adulterous generation asks for a miraculous sign! But none will be given it except the sign of the prophet Jonah. 40For as Jonah was three days and three nights in the belly of a huge fish, so the Son of Man will be three days and three nights in the heart of the earth. 41The men of Nineveh will stand up at the judgment with this generation and condemn it; for they repented at the preaching of Jonah, and now one[a] greater than Jonah is here.
1The Pharisees and Sadducees came to Jesus and tested him by asking him to show them a sign from heaven.
2He replied,[a] "When evening comes, you say, 'It will be fair weather, for the sky is red,' 3and in the morning, 'Today it will be stormy, for the sky is red and overcast.' You know how to interpret the appearance of the sky, but you cannot interpret the signs of the times. 4A wicked and adulterous generation looks for a miraculous sign, but none will be given it except the sign of Jonah." Jesus then left them and went away.
29As the crowds increased, Jesus said, "This is a wicked generation. It asks for a miraculous sign, but none will be given it except the sign of Jonah. 30For as Jonah was a sign to the Ninevites, so also will the Son of Man be to this generation. 31The Queen of the South will rise at the judgment with the men of this generation and condemn them; for she came from the ends of the earth to listen to Solomon's wisdom, and now one[a] greater than Solomon is here. 32The men of Nineveh will stand up at the judgment with this generation and condemn it; for they repented at the preaching of Jonah, and now one greater than Jonah is here.
The ninevites were a perverse and adulterous people and was the center of worship of Ishtar. Ishtar was a goddess of love, war, and sex. Ishtar is most associated with sexuality and sacred prostitution.
1 The word of the LORD came to Jonah son of Amittai: 2 "Go to the great city of Nineveh and preach against it, because its wickedness has come up before me."
3 But Jonah ran away from the LORD and headed for Tarshish. He went down to Joppa, where he found a ship bound for that port. After paying the fare, he went aboard and sailed for Tarshish to flee from the LORD.
4 Then the LORD sent a great wind on the sea, and such a violent storm arose that the ship threatened to break up. 5 All the sailors were afraid and each cried out to his own god. And they threw the cargo into the sea to lighten the ship.
But Jonah had gone below deck, where he lay down and fell into a deep sleep. 6 The captain went to him and said, "How can you sleep? Get up and call on your god! Maybe he will take notice of us, and we will not perish."
7 Then the sailors said to each other, "Come, let us cast lots to find out who is responsible for this calamity." They cast lots and the lot fell on Jonah.
8 So they asked him, "Tell us, who is responsible for making all this trouble for us? What do you do? Where do you come from? What is your country? From what people are you?"
9 He answered, "I am a Hebrew and I worship the LORD, the God of heaven, who made the sea and the land."
10 This terrified them and they asked, "What have you done?" (They knew he was running away from the LORD, because he had already told them so.)
11 The sea was getting rougher and rougher. So they asked him, "What should we do to you to make the sea calm down for us?"
12 "Pick me up and throw me into the sea," he replied, "and it will become calm. I know that it is my fault that this great storm has come upon you."
13 Instead, the men did their best to row back to land. But they could not, for the sea grew even wilder than before. 14 Then they cried to the LORD, "O LORD, please do not let us die for taking this man's life. Do not hold us accountable for killing an innocent man, for you, O LORD, have done as you pleased." 15 Then they took Jonah and threw him overboard, and the raging sea grew calm. 16 At this the men greatly feared the LORD, and they offered a sacrifice to the LORD and made vows to him.
17 But the LORD provided a great fish to swallow Jonah, and Jonah was inside the fish three days and three nights.
Here we see Jonah offering himself up as sacrifice to save those on the ship. Being thrown into the water is symbolic or a type of baptism. This baptism occurred prior to beginning his preaching ministry. The extraordinary arrival of Jonah certainly grabbed the attention of the ninevites. They heard the Word of God, repented, and God forgave their sins and spared them.
Again Jesus had an extraordinary arrival as seen in the nativity narratives. Prior to beginning is public ministry preaching repentance Jesus was baptized, and immediately the sign of Jonah was present in the form of the Spirit of God as a dove.
Just as Jonah spent three days and three nights in the belly (middle) of the whale and was resurrected Jesus spent three days and three nights in the heart (middle) of the earth and rose from the dead.
Those who heard and heeded the Word of God proclaimed through Jonah were saved. Those who hear and heed the Word of God (Jesus) will be saved.
In short, the Book of Jonah points to Jesus Christ. Jonah points to a message of God's unmerited favor (grace) on all those who believe and trust Christ for salvation.