Is there a sin upon one who performs Qunūt in the last rakʿah of every prayer, thinking it permissible to do so, and what is incumbent upon him to do?

You are not to be blamed for your past perseverance in reciting Du'a al-Qunut due to your ignorance. However, in the future, you should not recite the Qunut except for a valid reason (calamities befalling Muslims). Persistently reciting the Qunut in all five prayers is an anomalous opinion, for the Prophet, peace and blessings be upon him, recited the Qunut for a reason and then abandoned it when that reason ceased to exist. This is the view held by the jurists among the people of Hadith (definition).