What is the role of faith presuppositions related to whether the Gospel of Mark was embellished after the close of the apostolic era?

If the Gospel of Mark contains prophecies fulfilled well after the second century, that would negate the argument about accurate prophecies. I have not undertaken a study of the prophecies of Mark, so cannot say whether it has any, but I suspect that it does. My confidence is based on the Gospel of Matthew, which contains many detailed prophecies that came to pass in the centuries following its composition.

A second line of reasoning has to do with hidden structural patterns. If patterns are present in the gospel that were not known or reported on during the first centuries of the church but have been discovered recently, then those patterns act like anti-counterfeiting measures. If the patterns are extensive and undisturbed, material could not have been added, subtracted or altered without detection. Again, I have not studied Mark's structure, but have done the work for Matthew. Matthew has a seven-part pattern that occurs at least seventeen times in the gospel, at different scales, from the overall structure of the book to sections comprising multiple chapters down to verses of a single chapter. These overlapping and interlocking patterns (sometimes coupled in forward-reverse pairs to form chiasms) preclude all but the simplest modifications from being made undetected.

Far and away the most common faith belief relevant to the dating of Mark is the presupposition espoused by the Tubingen school in the early 19th-century, and continued among naturalistic scholars today: the dogma that Christianity is a fraud.

Without this particular anti-Christian religious belief, most of the arguments for late dates or drastic embellishment of Mark (and the other canonical Gospels) lose all force.

Let us consider the following comparisons:

Prophecy

• Dogmatic belief: there's no such thing as prophecy, therefore the Olivet discourse in the Synoptics must have been written after Jerusalem was destroyed in AD 70
• Historical evidence: the destruction of Jerusalem was predictable in advance (we know this from Josephus--see Wars 6.5.3); the specific language used in the Gospels to describe the destruction of Jerusalem draws from the Old Testament & 1 Maccabees. No religious beliefs for or against prophecy are needed to admit the possibility that the fate of Jerusalem was predictable in advance.

I explore the circularity of the "dogmatic belief" further in my video here.

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Quotations

• Dogmatic belief: any quotations of the Synoptic Gospels prior to Justin must be fabrications or quotes from hypothetical documents (this assumption is necessary to preserve desired conclusions)
• Historical evidence: Matthew, Mark, and Luke are all quoted by Polycarp in his Epistle to the Philippians, written ~AD 107. Matthew is quoted extensively by other writers as well in the late 1st and early 2nd centuries.

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Gospel embellishments

• Dogmatic belief: the beliefs about Jesus circulating among Christian leaders at the end of the first century were markedly different from the beliefs about Jesus held by the apostles in the early 30s.
• Historical evidence: though there are textual variations in the manuscript histories of the Gospel texts, nothing of the magnitude imagined by those dating the Gospels to the 2nd century is anywhere to be found. Textual modifications at this level do not exist in the manuscript evidence, they exist only in the imagination.

(I demonstrate the circularity of the dogmatic belief here).

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Who is reliable

• Dogmatic belief: Marcion should be trusted more than Papias. Because Papias made an argument in favor of the reliability of a Christian text, he must necessarily be wrong (this is called begging the question).
• Historical evidence: Polycarp of Smyrna was a disciple of the Apostle John and one of the pillars of early Christianity. Polycarp's own student Irenaeus reports that Polycarp & Papias were colleagues, Irenaeus has respect for Papias, whereas Polycarp & Irenaeus vehemently rejected Marcion as a fraud. Marcion's contemporaries were in a position to fact-check Marcion and they rejected him. Papias' contemporaries were in a position to fact-check Papias and they accepted him.

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The Intellectual Rigor of Justin Martyr

• Dogmatic belief: Justin, the skilled apologist arguing before Romans who had the power to kill him, knowingly relied on sources that were written within his own lifetime (2nd century), but misrepresented those sources to his critics (by claiming these sources were apostolic documents), who were too dim to notice he was lying to them.
• Historical evidence: If the Gospels were based upon the writings of Marcion, Justin was in a position to know. That Justin relied upon Matthew, Mark, and Luke as authentic, non-Marcionite sources--when his life was on the line--is very good reason to believe that Matthew, Mark, and Luke are not based upon the work of Marcion.

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Hypothetical documents

• Dogmatic belief: Hypothetical documents (or communities) which support the belief that the Gospels are late, unreliable propaganda are documents (or communities) that should be accepted & recreated, regardless of the absence of evidence for them, because they support a desired conclusion.
• Historical evidence: Q, Proto-Mark, Deutero-Mark, Proto-Matthew, Proto-Luke, and many other creative inventions are supported by zero historical, manuscript, or quotation evidence. Furthermore, I argue on my channel that it is possible to explain the Synoptic Phenomena without appealing to hypothetical documents.

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Conclusion

Those who wish to discredit the Gospel of Mark (or the other canonical Gospels) appeal to all manner of dogmatic beliefs in order to support the preferred outcome that Christianity is a fraud.

Those willing to look at the evidence with an open mind will find ample reason to believe the Gospel of Mark was written prior to Paul's arrival in Rome (~AD 60). My argument in response to the question linked by the OP, making the case that Mark was written approx. AD 55, is entirely secular in nature. No theological or dogmatic premises are needed. We only need acknowledge that the historian Clement of Alexandria--one of the greatest Christian scholars of the 2nd century--is a more reliable source for first century history than are those wishing to cause a stir in academia 2000 years later.

Those who believe that the Gospel of Mark is a reliable historical document do not need to appeal to any religious arguments to show that the Gospel of Mark was written at a time when:

1. The author was in a position to know what he was talking about
2. The audience was in a position to reject Mark if it was unhistorical

If they wish to make additional, theological claims about Mark, these claims would need to rely on theological premises. But the date of the composition of Mark does not.

Appendix--response to History Stack Exchange

Among the posts on the linked question on History Stack Exchange, and the comments upon them, are several questionable assumptions:

• The idea that Marcion wrote his gospel at the age of 5 is not credible
• The proposed solution to the Synoptic Problem in the currently most upvoted post not only fails to address the argument from order and the cultural milieu presupposed by the authors, but actively argues against what can be derived from the data (examples here & here)
• Papias was not writing circa AD 130--this is based on a known, historical error by the unreliable writer Philip of Side. Excluding one erroneous statement by Philip, all evidence points to Papias writing no later than the first decade of the 2nd century.
• All manner of hypothetical documents--for which there is no historical, manuscript, or quotation evidence, are appealed to...in an effort to avoid the statements made by prominent early scholars (e.g. Clement of Alexandria or Origen of Alexandria) who did not need to rely on speculation to report the history of their era.
• The appeal to major embellishments of Paul's epistles is supported by 0 manuscript evidence.

Luke

One of the objections raised was: "Luke drew from Mark, so Mark has to be earlier than Luke, but Luke's date is very uncertain, possibly as late as 110 CE."

I reject both assertions here:

• The belief that Luke drew from Mark is popular, but the popularity came from arguments that have been decisively rebutted. The credibility of the arguments is gone, what remains is the belief they created. Each of the major arguments for Markan Priority is circular, or reversible, or both. I respond to each of them in my video Why I Do Not Believe in Markan Priority.
• That Luke may have been written as late as AD 110 is not tenable unless the dogmatic beliefs I've listed above are accepted. Colin Hemer has demonstrated exceptionally well that the Book of Acts (which followed Luke) was written in AD 62 (see here), offering a solid upper bound for Luke. It is possible to argue for a lower bound for Luke's Gospel of AD 49, but an extensive argument to that effect would probably need to be its own question.