What is the earliest copy of a Gospel we have? Second Century? How do you know that the copies we have are the same as the originals?
When I came into this chapter of the thread, I said it was my belief that it can be demonstrated that the order of writing of the synoptics is Matthew, Luke, Mark. That means the question you ask should be asked of Matthew. I also said that it's possible to identify the bits in Matthew that have been edited in as distinct from those which belong to the original narrative. I mentioned the 20-odd doublets, but there are other many other bits that look as though they interrupt the continuity of the narrative. Also some bits appear to be written in a different style or using words not characteristic of the main narrative. (By the way, that doesn't mean these bits aren't true).
Scribes copying the originals for circulation may have changed words or made mistakes here and there. But when you have coherence in the underlying narrative, it's logical to believe that what we have is essentially the same as what was in the original of that underlying document, with bits edited in - similar to what looks to have happened at the end of Mark.
When it comes to the later two, Luke and Mark, it appears that Luke has used Matthew as a template and rewritten parts of it in his own style, and also added material from his other sources. Mark has conflated Matthew and Luke, and expanded parts of it for dramatic effect, adding details of his own.
So in a sense I agree with you that the original has been changed over time, but I believe
the synoptic trilogy gives us the record of the changes.
As an example to prove it, take the healing of the paralytic, told by Mark. It's undeniable that he has reworded Matthew and Luke at one point: he has lifted the phrase 'to the paralytic' out of its natural context as used by the narrator in Matthew and Luke, and inserted it into direct speech where it doesn't belong. Similarly, the phrase, 'take up your mat' belongs with 'and go home' but Mark lifts it out of that context and inserts it where it doesn't belong - 'rise, take up your mat and walk'.
These are undeniable evidence that Mark has made changes to the other two.
So I think we can put to bed the idea that we don't have an accurate copy of what was originally written.