No it isn't a nuclear option - it is the appropriate starting point for academic scholarship. To look for corroborating evidence, whether that be independent reports or perhaps archeological evidence.
No, your proposal says no document where the originals have been lost has any value unless the archaeology supports it. There's no archaeological evidence for a lot of the events in ancient history that we know only from documents. Was Socrates a real person? What archaeological evidence backs up Herodotos?
You also have to ask questions about the transmission of the material generation to generation - some routes will be more robust than others. So, for example, verbal transmission is likely to be the least effective in preserving the original.
By definition a written document is not transmitted orally. I think Mark was probably working from oral sources but the variation - or lack thereof - in the copies we have suggest that they all came from a single written source.
Also you need to ask question about motive during transmission - someone with an agenda or effectively using information as propaganda is likely to be more suspect than someone with no axe to grind.
And we see examples of that: the ending tacked on to Mark would be the obvious one. But scholars seem to be able to identify the bits that are suspect.
You also need to ask about the significance of accepting or not accepting the writing as close to original. Undoubtedly there are elements of Caesar's accounts (whether written by Caesar or someone else and/or substantially altered later) which are propaganda and hyperbole.
Well since the motive for writing the Gallic Wars was Caesar's aggrandisement in Rome, we can be sure that some of it s propaganda and hyperbole, but that is not the same as claiming we don't have any idea of what he wrote, which is your claim.
the details are of limited consequence.
Quite. And they are of limited consequence wen discussing the gospels. This is really an academic exercise. Does it matter in the great scheme of things whether Matthew or Mark was written first? No not really. Does the date of the gospels matter? Again, not really. If Spud proved that the gospels were written in the 30's we would still not accept the claims of Christianity. We might assign more credibility to the non supernatural elements of the story and the Jesus mythicists might have to eat their words but we wouldn't say "oh yes, Jesus was resurrected".
1. what we have now (and in early fragments) represents what was originally written (we do not know this)
We do not know for certain, but we are confident we have something approaching the originals.
2. that they were written before the temple destruction and therefore represent a real prophecy (except possibly for Mark most scholars reject this and even for Mark we cannot know whether an original written perhaps a couple of years before the temple destruction contained that section)
Mark has that section. Perhaps there was an earlier version without that section, but it's gone if it existed. The version of Mark that had that section was used by both Matthew and Luke as a source. So I think we can provisionally accept the section being in early versions of Mark, although I would agree that they probably date from after the destruction of the Temple or close enough to its destruction that it was highly probable that it was going to happen when Mark was written.
3. and therefore the gospels should be trusted (presumably including all the other claims)
[/quote]
I think there's good reason not to trust the content of the gospels even if we prove that we have verbatim copies of the originals. I think there is enough internal and external evidence to demonstrate they are not reliable at all without having to question the modern reconstructions of the text.
It is therefore critically important to be confident about what was in any pre-AD70 gospel (if one even existed), yet we cannot know that.
No it isn't. If it could be proved that somebody in 40CE predicted the destruction of the Temple in 70CE it doesn't mean they were the son of God any more than my prediction in 2015 that there would be a global pandemic proves I am the son of God. It does invalidate Spud's argument but Spud's argument doesn't meet the standard for such extraordinary claims anyway.