Does teaching five people to fast on Ashura, and their fasting it, result in the teacher being forgiven for five years, or is the forgiveness limited to one year for his fasting alone, in addition to receiving the reward for their fasting?

"Whoever guides to good is like the one who does it." The meaning of "like" here is that the one who guides receives a reward for the guidance, just as the doer receives a reward for their deed, but it does not mean their rewards are equal. Therefore, your guiding five people to the reward of fasting the Day of Ashura entitles you to a reward, but it does not mean you will receive a reward equivalent to their fasting or the forgiveness of five years, as there is no evidence for that.