We assume from your inquiry that you are referring to Christian leaders who have either been falsely accused of crimes or who have been in violation of the laws of the land in which they were living that suppressed religious freedom. We know from biblical as well as historical accounts, that since the days of the apostles persecution and imprisonment of true followers of Jesus have been implemented by the Roman Empire, the Holy Roman Empire, civil governments run by monarchies, dictators, communist and Islamic regimes, and the like.
If a true Christian leader has been imprisoned because he was either falsely accused or was not afraid to profess his faith, then, in answer to your first and last questions, he is still worthy of leadership even though imprisoned. We have the example of the Apostle Paul who, under house arrest in Rome, boldly preached about the kingdom of God. (See Acts 28:17-31.)
How many true Christians have been arrested in the past and how many will be arrested before this present age comes to an end? We do not know. However, we do know from history that many thousands of faithful Christians were arrested and martyred for their beliefs during the Dark and Middle Ages as well as during the Reformation. Today, persecution of Christians from all denominations is on the rise. In October 1998, the Oregonian newspaper did a five part, in depth series on the persecution of Christians worldwide, focusing on the plight of the believers in Pakistan, Burma, Sudan, Egypt, and China. Since that article was published, the frequency of persecutions has increased.
Will the rate of Christian arrests and persecutions continue to increase before this age comes to an end? In all likelihood we believe it will. “Night is coming, when no one can work.” (John 9:4) We venture to guess that Christians who will be arrested and/or persecuted at the very end of this age may be the ones who profess the truth of the kingdom message openly in a hostile environment.