It is not believed to be an eyewitness account due to his amount of Mark used. He doesn’t seem to be telling his own story but correcting Mark’s.
If you believe in Marcan priority, of course.
If you make the claim of Matthean priority, on the other hand, you can assert “eyewitness” for both.
(And, in any case, given that there
is original material in Matthew, even when correspondences are taken into account, it’s not necessary to claim “it is not believed to be an eyewitness account.” In fact, many scholars
do make the “eyewitness” claim, so it would be more accurate if you asserted “many do not believe Matthew to be an eyewitness account”…

)
We don’t have a copy of the Q document.
You don’t really need a document, do you? The claim is of a
source, not a
document, per se, anyway… right? If you have sayings of Christ that are well-known and often-quoted, then it’s trivial to imagine them being used in the development of a Gospel.
He used over 90% of Mark.
Or… 90% of Mark comes from Matthew. Listing correspondences is easy; discerning ‘source’ and ‘destination’ takes more effort and requires some assumptions…
A document that is used by Mt and Lk is a lot more likely than testimony from Mary, James etc. without being acknowledged.
Umm… Luke
does have an acknowledgement, right?
What purpose was there, in deleting the prayer that was in his source?
All of this talk of “deleting” and “inserting” still
presumes a particular order!
