1. I know this how? Try Google. There are so many sources available that they may even satisfy you.
2. There weren't quite a few. There were none. If the Jews were chased by force out of Rome and the Christians were illegal, there weren't any, at least operational.
3. Christ sent his disciples (including all Christians) into the world of pagans. By word or deed or posting, we see those who would remain so.
Find the earliest. The first time we hear about an upside-down crucifixion is in the apocryphal Acts of Peter written about 170 - 190 AD. That is where the myth got started.
2. There weren't quite a few. There were none. If the Jews were chased by force out of Rome and the Christians were illegal, there weren't any, at least operational.
What does it take to operate a cemetery --- a shovel? And look on a map sometime of Rome and see all the Jewish cemeteries --- I wonder how ancient some of the old grave markers are?
Jews who were Roman citizens, like Paul, could still enter or remain in Rome even after Claudius's decree, but those who were not citizens, like Peter, would not even be permitted to enter. In the Book of Acts Paul meets with a group of Jewish elders when he arrives there in 60 AD. These must have been part of that community of Jews who were permitted to stay in Rome.