What seems strange to me about this whole affair is that, according to various scholars, the dead sea scrolls proved that the book of Isaiah had hardly changed at all in about 1,000 years of writing
This is a bit of an unequal comparison. Isaiah actually had a rather complex early history, with quite large accretions (e.g. Deutero-Isaiah, Trito-Isaiah, other interpolations) which can be demonstrated through literary analysis. But the text reached a fairly stable state within a century or two (i.e. in the fifth century BC), and the Great Isaiah Scroll (1QIsa) dates only as far back as 100 BC. That the MT exhibits minor deviations 1,000 years later attests to the fairly subsequent stable nature of the text, but does not shed light on the book's early literary history. Other books in the Dead Sea Scrolls, such as 1-2 Samuel and Jeremiah, show major deviations from the MT (this fact is less often mentioned). But in the case of the gospels, we're talking about the early literary history where there was much redaction. This is analoguous to the early redaction of Isaiah (i.e. from the seventh century-fifth century BC). The gospels by and large had a more stable textual history centuries later (tho this is relative since there was still much textual variation).
And it seems that the other synoptic gospels borrowed heavily from Mark.
I think as much as 80% of Mark is in Matthew.
It just seems just as much an article of faith to brand the entire gospel accounts as complete mythological fictions than to accept that much of them may indeed be true.
Each matter should be investigated on a case by case basis. I would not say "entire gospel accounts" are "mythological fictions". But if you look at how the stories were composed and especially how they are derived through haggadaic reuse of OT texts (a modern reader may well anachronistically call it plagiarism), just like scores of other haggadaic stories were composed at that time, one has to come to a judgment of how much in the gospels pertains to historical memory and how much derives from OT texts reinterpreted and recycled in new ways. And any value judgments one makes must take into account the lack of the modern distinction between fiction and "history" in ancient writings, the ancient perception of fiction, and the use of fiction by Jesus himself to give moral teaching (via parables). How much are these stories to be regarded as parables with moral lessons (with Jesus as the primary character), tho they are not historically accurate? JD Crossan's distiction between mode and meaning is especially pertinent here. Regardless of what mode the story is in, what is its meaning for us?
Is it not the case also that the early church fathers going back to the second and third centuries quote extensively from the gospels as we read them today?
Some do, some don't. Many of the early ones don't use the gospels per se but gospel traditions floating around in oral tradition that varied in literary form. So Ignatius, for instance, tells a story of a christophany to the apostles that is similar to what is found in Luke and John, but is clearly independent of them. And the author of 2 Clement takes independent sayings that have different narrative contexts in the synoptic gospels and uses them to construct a new dialogue between Jesus and Peter. Papias had other traditions that he drew from oral tradition that are not in the gospels. In fact, he said he preferred the living words attributed to Jesus in oral tradition than the written words of the gospels. In one instance, he described a conversation between Jesus and the apostles (including Judas) that is clearly derivative of the Jewish apocalypse of 2 Baruch, putting the words into the mouth of Jesus. Later writers clearly used our four gospels. But some did not use our gospels per se but harmonies of them, in which the gospels would be freely stitched together and edited into a single work. Justin Martyr used a harmony, and Tatian borrowed it to form the basis of the Diatessaron.