There is nothing that could happen in the Churches that isn't internal. Ministers, in many cases, preach a social gospel. They do not discipline Church members. Ministers themselves act like unsaved people. What is happening in the Churches is a lack of leadership to guide the Christians who need someone to guide them. That is the job of pastors.
Sheep are easily mislead - that is why the bible compares people to sheep. Sheep are stupid and so are people. It is only be becoming familiar with the Truth that one can recnogize a lie. You are right that many ministers are preaching heresy and political correctness - when they should be teaching their flocks to love Jesus Christ AND OBEY HIS COMMANDS. Many mainline churches are essentially apostate there is no question about that. Going to church does not make one a Christian any more than going to Germany makes one a German.
This is incorrect. The problem is that sick culture (an external influence) is being brought in the churches by corrupt leaders. You can't possibly believe that the churches thought up this stuff (pro-homosexuality, feminism, etc.) on their own.