Don't agree with any of that. It isn't reasonable to assume that they are reliable and they aren't evidence enough. Common sources and copying of each other doesn't tell us that.
My view, from my own study of the Synoptics, is that Matthew wrote a narrative that has two hinge points involving the imprisonment and execution of John the Baptist, after both of which Jesus withdrew: firstly, after John's imprisonment, to Galilee (4:12); then, after John's execution, he withdrew outside the territory of Israel (14:13, 15:20).
Matthew is a source for Luke, who rearranges some of Matthew's material to suit his purposes in writing. The most obvious evidence for this is that whenever Luke is following Matthew's chronological order, he gives a definite time marker, such as "the next day" or "then" (eg 9:37, 9:46, cf Mt 17:14, 18:1) . But when Luke is not following Matthew's order, he uses a vague time marker such as "on one occasion", or "one day" (eg Luke 5:1, cf Mt 4:18; Lk 9:18, cf Mt 16:13).
Luke also uses other sources.
Mark uses Matthew and Luke, and adds details of a few incidents that are not found in Matt or Luke. The main evidence for this is that whenever Mark stops following the order of either Matthew or Luke, he starts using the order of the other. This is consistent with him having a scroll of Matt and Luke in front of him and working his way through the two scrolls, taking what he wanted from each. It would be very unlikely for Matt to always return to Mark's order at the same point that Luke departs from it, and for Luke to always return to Mark's order when Matthew departs from it.
It is also evident that while Luke and Mark use Matthew as a source, they don't follow his thematic structure. For example, in Matthew, Jesus' withdrawal to Galilee after John's imprisonment fulfills Isaiah's prophecy of a light dawning on the people (in 'Galilee of the Gentiles') living in darkness, Isaiah 9:1-2. But for Luke, Jesus' return to Galilee after his baptism is simply the beginning of his ministry. Mark follows Matthew in noting that Jesus went into Galilee after John's arrest and follows Luke in saying that this was when Jesus began his ministry; but Mark omits Matthew's reference to Isaiah 9 and so, like Luke, misses the relevance of the ministry beginning in Galilee.
Hopefully the above is not too brain-bending, I've tried to explain it in order to show how it is enough to convince me that the gospels as we have them are accurate reproductions of what was originally written. For more details see Harold Riley's books on the Synoptic gospels.
It is likely, in my opinion, that a crucifixion happened but that the victim as buried in a tomb is much less likely given what we know of the practices of the time. Since we don't have any early copies of the Gospels we don't know what was written down during the lifetime of the people involved and we don't know if attempts were made to debunk the stories. The accounts we have are from long after the event, written by unknown authors who were believers and who were not writing independent history but were presenting accounts which reflect their beliefs.
As I've said before, the gospel of Matthew shows signs of having been written before AD70, when the temple was still standing and the Jews still practiced their religion Jerusalem; for example it mentions the temple tax, the way the pharisees would pray on street corners to be visible, or the note to the reader to understand Jesus' warning to flee from the city when they see the abomination of desolation. So Matthew at least was written within living memory of the events. Luke claims that his sources were those who witnessed the events (1:2), and John's author claims his source is one of the disciples (John 21:24).
It wouldn't necessarily need more than one or two people to have experiences following Jesus' crucifixion that they believed were encounters with a risen Jesus (exactly what form that took we don't know), remembering that post grief experiences where people see and talk to dead loved ones during a period of grief are a known phenomena, for stories to begin to spread on a small scale initially at a time when it wasn't possible to know where Jesus' body was. Over time with retelling the stories become exaggerated with features added. At some point they were written down then later authors used those writings and copied each other to end up with the current Gospels. Paul also has an experience he believes is Jesus (a vision?). An alternative scenario which leads us to the Gospels we have today.
True, but again the evidence shows that the gospels were written during, or soon after, the eyewitnesses' lifetimes (certainly pre-AD70 - there is no mention anywhere of the destruction of Jerusalem having occurrred). Interestingly, Luke selects his accounts of the resurrection appearances with a different purpose in mind from the other gospel writers: he wants to focus on the necessity of Christ's death and its fulfillment of scripture. Hence the different form taken by Luke in his conclusion to that of Matthew. Mark (as he has done throughout his gospel) ends up, for his conclusion, conflating Matthew, Luke and his other sources, among which may be John's gospel (cf Mark 16:9. John 20:16)
