Must a Muslim know all the astray sects and their rulings for fear of not declaring as disbelievers those whom the people of knowledge have declared as such?

A Muslim is not obligated to know all deviant sects. Rather, he is obligated to know the truth and follow it. Knowing something of falsehood to avoid it and warn against it is commendable. The rule "Whoever does not declare a disbeliever a disbeliever is himself a disbeliever" applies to disbelief that is known by necessity to be part of the religion, such as the disbelief of Jews, Christians, and Zoroastrians.