How is it to be determined, in a Catholic sense, that certain parts of scripture are metaphorical?
This isn't exactly an answer to your question, but it demonstrates how scripture should be accepted based on the literal first, metaphorical if irreconcilable with the literal, and lay persons must always submit to the Church's opinion, ultimately.
Revelations 19:7
Let us rejoice and be glad and give the glory to Him, for the marriage of the Lamb has come and His bride has made herself ready.
Now, while its obvious to all that Jesus isn't actually a bleating lamb, the metaphor is clear to Catholics that Jesus is the sacrificial lamb of God who takes away the sin of the world. That we eat the lamb in Communion continues the metaphor, yet, it would be false to say that we should not literally eat Jesus because the passages above are metaphorical. In other words, the metaphor strengthens the whole picture, it doesn't deny the whole picture.
Second Vatican Council in the "Dogmatic Constitution on Divine Revelation," section 8:
[The doctrinal tradition], which comes from the apostles, develops in the Church, with the help of the Holy Spirit. For there is a growth in the understanding of the realities and the words that have been handed down. This happens through contemplation and study made by believers, who treasure these things in their hearts (cf. Lk 2:19, 51), through the intimate understanding of spiritual things they experience, and through the preaching of those who have received through episcopal succession the sure gift of truth. For as the centuries succeed one another, the church constantly moves forward toward the fullness of divine truth until the words of God reach their complete fulfillment in her."
Doctrinal understanding properly can be said to "develop" in the Church in two main ways: by
clarification of expression, and by
elaboration of content. Not by adding to or taking away from let alone contradicting. Here are some Scriptural passages for support of the Immaculate Conception.
Gabriel referred to Mary as "full of grace" (Luke 1:2)
Elizabeth: Luke 1:42: and she exclaimed with a loud cry, "Blessed are you among women, and blessed is the fruit of your womb!"
Luke 6:43:"For no good tree bears bad fruit, nor again does a bad tree bear good fruit;"
1 Corinthians 6:17: But he who is united to the Lord becomes one spirit with him"The above is not terribly convincing support for the Immaculate Conception, but these passages are some of the best biblical offerings we Catholics have. There are others, but they aren't much better at pointing out that Mary was without sin unless paired with revelation from Tradition. As it turns out, the scriptural passages for flat earth collectively are far more certain when one considers that God imparts to us, as PD teaches, in a way so that we can see and understand. Scripture says repeatedly that earth has four corners, ends, pillars, has a dome-like vault above it, with water above that; That Christ was shown the entire earth with all its kingdoms. Now, while this last one might be a mystical type vision suggesting metaphor, the book of Daniel suggests again that he saw the entire earth, something possible only if earth was not a globe: Daniel 4:10-11 10This was the vision of my head in my bed: I saw, and behold a tree in the midst of the earth, and the height thereof was exceeding great. 11The tree was great, and strong: and the height thereof reached unto heaven: the sight thereof was even to the ends of all the earth." Doubters insist, "well that was a dream". Yet nothing about the dream is reconcilable with a globe and clearly suggests a planar earth.
When joined with other passages, literally dozens, one gets the same picture. Earth is described as simply as it sounds, flat, with mountains and valleys, but never even remotely does it suggest its globular. Add to that the statements of the Fathers and exegeses from the saints, one would find himself outside the ridiculous to try to make a globe earth fit, never mind the science. The final straw is that the globe has been promoted throughout the ages by pagans and is accepted by pagans today. Modern science lies about the globe and remains unable to prove it. It even turns out most of NASA proofs are lies as they attempt to show photos of a globe earth. Sometimes, God intends for man to get a clue before He reveals more. In this case, man refuses to get a clue, and the grace of knowing appears to have largely been withheld. The people who sneer and say, "Where does it say in scripture that earth is flat?" are the same kind that say, "Where in scripture does it say Mary never sinned?" Well, its there; in scripture, in tradition, supporting a multitude of other proofs besides.