What is the ruling on the fasting and prayer of one who believed for many years that something کفْر (disbelief/apostasy) was permissible, based on the saying: "The اختلاف (differences of opinion) among scholars is a mercy, and it is permissible to adopt the view of any one of them"?

A Muslim is not to be declared a disbeliever merely by performing an act of disbelief or holding a belief of disbelief, due to the possibility of their ignorance of the ruling or the existence of a doubt preventing their excommunication. This is until the proof is established against them and the doubt is removed. If they persist thereafter, they are then judged to be a disbeliever. If you held a belief of disbelief out of ignorance, or misinterpretation, or due to a doubt, you are not required to make up for Ramadan (definition).