“Why were the Romans, so opposed to Christianity, so tolerant of Mithracism, which was from the arch-enemy Parthia?”
Christianity forbids worship of any other God. The Christians would not worship Caesar.
The Romans I understand were very pluralistic, as long as you bowed the knee to Caesar.
We still struggle with the relationship between church and state.
You are correct. Some pantheistic Romans even has a statue of Christ, along with their other gods. But the Christians and Jews would not sacrifice a pinch of incense to Caesar, and were traitors. The Romans had grown up in contact with the Jews, so they left them alone, once they had destroyed their country.