What other field of inquiry uses "embarrassment" as at least one criterion to determine and what is and what is not a "fact" in anonymous and unprovenanced documents?
The field you mentioned in a recent post with approval, judges and jurors. Statements against interest are an exception to the hearsay rule (that is, in civilian "English" legal sytems, unsworn statements made outside the presence of the court are generally inadmissible as evidence about what the statement asserts, except under specified circumstances). There is also a provision for spontaneous declarations being admitted, which is very similar in practice to statements against interest.
In answering your question posed to the community, I am not endorsing the "criterion of embarasment" as it has been applied in any specific instance. Nor the hearsay rule, for that matter.
Are you saying that plausibility of a narrative is a reason we should believe it to be true?
Are you saying that plausibility is a reason to hold a narrative in equipoise? If not, are you saying that plasuibility is a reason to hold a narrative to be less likely true than not? If not to both, then what is the problem with holding a narrative to be more likely than not, according to its plausibility and in the absence of effectively countervailing evidence?
Because "Mark" made excuse on some facts & sayings which was not serving his theological/christological agenda. If these facts and sayings were not believed true, then there could not be invented because that would create confusion and doubt.
Whose confusion and doubt about what?
For example: why would Peter keep secret the title, Christ, he gave to Jesus?
The plot portrays Jesus on a suicide mission, but being killed is insufficient to achieve the mission's objectives. Timing matters, too. Secondarily, it heightens the dramatic tension of a key scene, Peter's denial in the priest's courtyard, that Peter not be at liberty to explain his actual relationship with Jesus. It's a further grace note that that cat is already out of the bag, but Peter doesn't know it. That is an example of a craft technique, the motif of the command which continues in effect after its usefulness has expired. A necessary condition to use the technique is to issue the command.
I could go on, but good writing is its own explanation. Why should "Mark" be embarrased that he is conspicuously skillful?
If "Mark" invented the wording of the sign on the cross, he would have "the Christ" instead
Why? Claiming to be the Christ isn't even a Jewish religious offence, much less a Roman capital crime. Assuming some wish to reflect a plain reading of Paul's epistles, Mark has to get a dead Jesus hoist by somebody on something wooden. A stoning followed by a gibbeting would work, but then who gets the "Truly a son of a god" line? A Jewish character? How would the Psalm 22 thing work?
(IN)RI solves some problems, and there are constraints not of INRI's making if a relationship with Paul as a source is to be maintained. It's a good solution, and plainly has in fact enjoyed a high degree of audience acceptance. It is thus well within the scope of shrewd invention by a master craftsman.