Marcion of Sinope was a real person who did have a significant effect on the church as a whole. However, you do slightly misunderstand what he said and did. Marcion was from Sinope in modern-day Turkey. He was not a 1st century person. Actually, he was born in the late first century, but his influence over the church began in the 130s AD. There is no evidence that Marcion did not believe Jesus was a historical person. In fact, I have not heard of a single person in the first several centuries with the audacity to claim that Jesus was not a historical person. It was not until modern times that anyone made such an audacious claim.
So, Marcion believed Jesus was real, but his description of Jesus was very far from that of the early church and the Orthodox position. He had beliefs similar to the Gnostics, but his differences with the Gnostics were almost as large as the similarities, and he is not considered a Gnostic. He definitely was a Docetist. Marcion believed that the God of the Old Testament was not a good God. He tried to largely de-Judaize Christianity and to disconnect it from the Old Testament entirely. As a Docetist, he believed that Jesus was real, but he only appeared to have a physical body. Docetist means appears. He denied the physicality of Christ. As the Gnostics, he believed that physical things are inherently evil. He not only rejected the entire Old Testament. He also rejected Matthew, Mark and John as too Jewish and he published the Gospel of Marcion, which was his edited version of Luke, which removed parts of Luke he felt were offensive. His canon was a truncated Luke and ten of Paul’s letters. He rejected the pastoral letters of Paul. In the 140s AD Marcion moved to Rome, where he began to teach his unorthodox ideas. He was promptly labeled a heretic. He returned to Asia where a remnant group continued to follow his teaching. His influenced had nearly disappeared by the beginning of the third century.
So, your thought that there was debate about whether Jesus was a historical person in the first two centuries is simply not correct. There was debate about whether Jesus occupied a physical body and was killed on the cross from the Gnostics and other Docetists such as Marcion, but there was no debate that Jesus was not historical. My answer to your first question is a solid no! Literally no one claimed that Jesus was a fiction/myth in the first several centuries. Given the number of eyewitnesses to the ministry of Jesus and, during the second century, the great number of those who had personally met eyewitnesses of Jesus made it impossible to sustain such a wild idea that Jesus was not a real person.
As for your second query, this is a good question. I believe that the principal reason for Gnosticism and Docetism was the influence of Greek philosophy and religion. To the Greeks, the idea that Jesus would feel human pain and emotion was blasphemy. The idea that God would suffer an agonizing and shameful death was fantastically difficult to conceive of, never mind accept. For the Greeks, Judaism was a tough pill to swallow, but Christianity was beyond the pale to the typical Greek philosopher, or most Greeks in general. There was a great temptation for early believers to syncretize their Greek philosophy and religious ideas with their Christianity. Docetism and Gnosticism were certainly not created by anything in the New Testament. That is for sure. It was not supported by any of the apostles!!! But there is a hint already in the first century of believers who were tempted to deny, not the historicity of Jesus, but his physical, human nature. In 1st John, the apostle John, near the end of the first century felt the need to describe Jesus as one we touched (1 John 1:1). These heretical ideas about Jesus were created by people who struggled to put their religious and philosophical background aside and, perhaps, who were intimidated by persecution from non-Christians.
Here is what you can be sure of. The Gnostics and Docetists were wrong. Plain and simple. The gospels, written by eye-witnesses or those who knew eye-witnesses utterly refute these two Greek-influenced ideas, and both Gnostics and Docetists were eventually declared heretics, as they should have been.