This is a continuation of my last post, where I began a critique of an article entitled 5 Reasons to Suspect Jesus Never Existed.
3. Even the New Testament stories don’t claim to be first-hand accounts.
This is true, but this doesn’t mean they can’t be trusted. Both Luke (1:3) and John (21:24) assure their readers that their Gospels are based on eyewitnesses. Bishop Papias (60-130AD) and other early church writers claim that Mark derived his story from the Apostle Peter. Those sound like pretty good sources.
The Two Marys at the Tomb, by Bartolomeo Schedoni
Modern scholarship rejects the consensus of the 2nd century church, that Matthew and John wrote the Gospel that bear their names. If the early church was correct, then two of the Gospel writers are first-hand accounts.
4. The gospels, our only accounts of a historical Jesus, contradict each other.
Sometimes the stories are different, but are these contradictions? For example, Mark claims that Jesus appeared to Mary Magdalene, while Matthew records that there was more than one woman. There is a difference between a partial report and a false report. In this case, Mark and Matthew are giving complimentary, not contradictory accounts.
Other differing accounts in the Gospels (most contain trivial details), can be explained in similar fashion. The early church recognized this.
5. Modern scholars who claim to have uncovered the real historical Jesus depict wildly different persons.
These scholars do not believe that the Bible is inspired by God, so any supernatural claims must be myths. These scholars cannot agree on what is myth and what is history in the Gospels, and they interpret these so-called myths in different ways. How could they possibly discover the same Jesus?
Skepticism is nothing new to Christianity. In the words of the Apostle Paul, “we preach Christ crucified: a stumbling block to Jews and foolishness to Gentiles, but to those whom God has called, both Jews and Greeks, Christ the power of God and the wisdom of God (1 Corinthians 1:23-24).”
The Other Six Days 
Your claim is that Mark and Matthew give complimentary, not contradictory, accounts, but unfortunately this claim does not comport with the evidence. These two gospels differ on whether the tomb was open when the women arrived, the identity of the person waiting inside (angel verses young man), on who Jesus first appeared to after the Resurrection (the Eleven vs. two disciples in the country), and on where he first met them (a mountain in Galilee vs. at a supper table). Both of these books have the angel telling the women to instruct the disciples meet Jesus in Galilee, but they differ from Luke which does not have Jesus even going to Galilee before his ascension, and Mark even differs from itself across chapter 16, with the traditional ending of the book given at verse 8.
These are valid points, but I don’t think you need to assume a contradiction between the Gospels because they develop their accounts differently and provide different details. Using your first example, Matthew (which is a later account than Mark), provides more details of the resurrection story than Mark by identifying the young man as an angel (who are often given human descriptions in the Bible), and explaining that this angel is the one who rolled the stone away as the women arrived and that soldiers were also present. A difference does not have to mean a contradiction.
There are no contradictions in reality, so when Luke has Jesus rising into heaven on the same day as the resurrection, and Acts has him ascending after “being seen of them forty days”, someone is getting it wrong. Atheists won’t be overmuch convinced that Jesus existed at least until Christians put forward a consistent account of the end of his ministry on Earth.