A good case can be made that Paul and his followers thought of Jesus as a mythical being. (A good case can also be made that his Jesus was thought of as an historical person.) The title of this thread is, "Did Jesus Exist?" (meaning did he exist on earth as a historical person), not "I know for sure that Jesus did not exist." I am not claiming to know one way or the other, but I think the preponderance of the evidence goes to the mythical interpretation.
This is the heart of the matter. You think that the preponderance of the evidence points one way, but to the vast majority of people, including non-Christians, that interpretation of the evidence looks very silly. Your case rests upon the assumption that prior to the writing of Mark, everyone believed Jesus to have lived in the heavenly realm, and that Mark invented the earthly life story of Jesus while knowing it was fiction. After this, everyone accepted Mark's version, including both the other gospelers and the enemies of Christianity. This is the part that most of us find quite silly.
To illustrate why, let's imagine a comparable scenario. Let's take someone who we all agree today did not live an earthly life: Frodo Baggins. Let's imagine that I write a book in which I assert the following things:
Frodo Baggins lived from 1932 to 1981.
Frodo Baggins was born in Mt. Sterling, Kentucky, and died in Baltimore, Maryland.
Frodo Baggins visited places such as the Empire State Building, the St. Louis Arch, and Wrigley Field.
Frodo Baggins met and talked with people such as Frank Sinatra, Richard Nixon, Garry Trudeau.
Frodo Baggins was tried by the United States Supreme Court on April 9, 1981, and executed three days later.
Now, do you think I would have any luck convincing people that my version of the life of Frodo Baggins was correct? Moreover, do you think that I'd be able to convince
everyone? Do you think that even people dedicated to proving my claims about Frodo Baggins untrue would accept that my main narrative about life was true?
If you answered "no" to any of those questions, then you understand why most of us have trouble taking your hypothesis seriously. You might try arguing that in ancient Rome there was less documentation then there is today and it would be tougher for people to double-check facts. That would be true; however, it would be a mistake to assume that ancient Rome was packed with superstitious people willing to accept anything. Richard Bauckham has shown in his book
What the Apostles Saw that people in the relevant time and place put a premium on getting accurate, eyewitness testimony.
So the questions you'd have to address to defend your hypothesis are:
1) Why would Mark choose to write a book about the earthly life of Jesus if everyone at the time knew Jesus had no earthly life? You've said that he did so to give hope to people, but isn't that a rather odd way to give people hope?
2) Why would anyone who knew Jesus to be a heavenly being switch to believing he was an earthly being based only on one source?
3) Why do we have no record of anyone in ancient times, even among enemies of Christianity, charging that Jesus did not exist?
4) If your sequence of events is plausible, why hasn't anyone else in history ever done the same thing, i.e. taken a story of a spiritual being and written a fictional story about that being's earthly life and somehow convinced everyone that their story was correct?