(ANNO DOMINI) is the Roman expression for: The year of the dominium of the Lord. This is to say that the "Lord" as a title was given to the highest office of the Roman Empire. At that time this was Julius Caesar and it related to any year he was in office.
There was a gate in Jerusalem, that had the statue of Caesar posted at the gate. It was expected that the Jews come into the city through that gate and pay their respect to the statue as their Lord and Master. After all, the Romans occupied the territory at that time, and also ruled over the Jews. The Jews avoided that gate like the plague, because they would have not been able to keep the commandment: 'Thou shall not worship other Idols..' The Christians did not have this problem.
Christianity really was promoted in Greece and spread from there. Remember that the world as it was known AT THAT TIME, was mainly ruled by Julius Caesar, and anything that was Roman was also in fashion. That is why the ANNO DOMINI was simply transferred to the God of the Christians by the Christians of that time as THEIR Lord.
Now, "B.C." stands for any year "BEFORE CAESAR". This is to say the Romans could not care less what came BEFORE CAESAR, and remember, the Romans ruled! Again, Christianity saw this as a golden opportunity to make this one of their own significance, by simply changing it to "Before Christ." Since the Greek word for king is "Christos", it fit. The Roman word REX or the Hebrew word MELEG would not have worked.
At that time the Christian movement was a small and insignificant sect. No one can seriously believe that the Roman Empire would have fixed something as important as the accounting of historic time and events to the Christian movement, especially if it was centered around a Jew to whom the Greek title of king was assigned, and whom they had crucified for rebellion against their own.
I think you would have to go to the library and look up works about Roman periods, especially under Julius Caesar. It was he who was determined to be worshiped as IDOL (GOD) by all nations of his time, especially those nations that he had conquered, and where his troops occupied the land. His role was not simply to rule, but to DOMINATE, hence DOMINI, and not simply a number of some sort of keeping track of time. Anything B.C (Before Caesar) did not matter to him one way or another. It was also coined to play down the achievements of Alexander the Great, (356-323 B.C) who was still in the people's mind at that time.