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5) Matthew the Apostle (an alleged eyewitness of the public life of Christ) would not have borrowed from a non-eyewitness when forming the basis of his account.

6) This indicates that (an alleged) Matthew the Apostle did not write the Gospel named after him. It must have been composed by an unknown person at a later date, using Mark's Gospel as a basis and adding additional material from other sources.

7) As the Gospel ascribed to Luke also improved on Mark's Greek, he must also have written late. This means the author could not have been a companion of Paul.

I agree with most of your post: poor Greek, Mark as disciple of Peter (scribe actually), Markian priority, etc. But let me expand a bit:

All of the Gospels were redacted (edited) by the sect that formed around each apostle-church.

The Gospel of Mark seems to be the most original, but since he was not an eyewitness, he had to depend on Peter's story.
Matthew (an eyewitness) probably wrote the non-canonical Gospel of Hebrews, on which a later anonymous redactor augmented with "Q", Mark (and maybe Thomas) to produce the canonical 'Gospel of Matthew' that we read now.
Luke (admittedly a non-eyewitness) was the physician to, and a follower of St. Paul, another non-eyewitness. The parts of Acts, Romans, Corinthians, etc. where Paul is featured is likely to be authored by Luke, and those texts were wove in with the other Apostle's stories by a (again) anonymous redactor to produce the final book. It's possible that Luke wrote a gospel that would've circulated among the Pauline sects and a later redactor would have "polished" it up, same as Matthew.

The 'Gospel of John' however, is a weird bird.
First off, the gospel is not of Apostle John of Zebedee but of John the Baptist (Jh 1:6, Jh 1:15, Jh 1:19-23, Jh 1:25-33), or rather of his followers that switched allegiance to Jesus (John tB didn't last that long).
Matthew and Luke include a birth narrative. John, like Mark, begins the story with the baptism of Jesus by John the Baptist. The other three gospels recount only one year of Jesus' ministry, John records three years, or at least mentions three Passover(s).
As the church was forming this gospel became controversial. That's because the gnostic sect that "owned" the text revered Mary Magdalene and believed she was responsible for the testimony. The early church had very strong opinions about women in the hierarchy and the idea of a woman in a position of power was unacceptable.
Here's a plausible explanation (which I accept) of how 'John' was redacted and thus became acceptable to the orthodox church.

9) As none of the authors of the Gospels were Apostles or their companions, their writings cannot be seen as accurate accounts of what (an alleged) Jesus said and did.

10) The authors must have been unknown writers, living at late dates, expressing their beliefs in the form of stories.

What I said!

You are so wrong and the facts you state are foolish.

John, like Mark, begins the story with the baptism of Jesus . . .

The Gospel of John begins with a restatement of Genesis 1:1a and goes on to assert an organizing principle solely responsible for the creation and sustenance of all things. It then details how this organizing principle took up human flesh for the purpose of fixing every malady associated with our first parents' choice to die.

Somehow the conspiracy of the early church was SO good it kept a cacophony of almost-Mark, Luke, Matthew, and John gospels from surviving for posterity in spite of a smorgasbord of religious practices and beliefs in the region. There are small variations of wording in gospels and disputes over placement and presence passages, but they don't show the weird kind of thinking that appeared in the post-year-100 pseudo gospels that have psychedelic things like crosses floating out of tombs. The four recognized gospels and known variants are very focused on Jesus and very realistic in tone.