Is Christianity based on older myths?

[Nope]

It also assumes that, presumably Paul, was familiar with the details of each of the examples of ancient myths you give. I don't see reasons why that would be true. He would have been studied in Jewish law and the OT. Access to the details of extinct and eastern religions would have to be explained for this to be plausible.

Well, he was from Tarsus. Are you saying that a native of that city would have been unfamiliar with the Roman cults?

Paul was a Roman citizen.

http://www.biblestudy.org/roman-empire/h...nship.html

Acts 22 alludes to two ways of becoming a citizen of Rome. We pick up the story with Paul's visit to Jerusalem's temple with four Jewish converts. Jews see him enter the temple and begin a riot. Roman soldiers save Paul from an almost certain death by beating and take him to a nearby barracks for questioning.

25. But as he was being tied with the thongs, Paul said to the centurion who stood by, "Is it lawful for you to scourge a man who is a Roman and uncondemned?"

26. Now when the centurion heard this, he went and reported it to the chief captain, saying, "Do you realize what you are about to do? For this man is a Roman." 27. And when the chief captain came up, he said to him, "Tell me, are you a Roman?" And he said, "Yes." 28. And the chief captain answered, "With a great sum of money I bought this citizenship." And Paul said, "But indeed, I was born free." (Acts 22)

One can become a citizen by either birth or buying the privilege. Paul's birth in a Jewish family occurred in the city of Tarsus within the province of Cilicia (Acts 22:3). Although a Jew, his birth in the city grants him citizenship. This is due to Tarsus' designation as a "free city" by Rome. The commander, however, had to pay a large sum of money to be a citizen.