Does the prayer become void if the worshiper believes that a Sunnah of the prayer is an obligatory act, such as believing that sadl (letting the arms hang down by the sides) is obligatory?

What has been mentioned about the invalidation of prayer due to believing a Sunnah to be an obligatory act is incorrect, and your prayer is valid. If you learned the description of prayer from a scholar and did not distinguish between the obligatory and non-obligatory parts, your prayer is valid as long as it is free from defects, whether you knew about the existence of obligatory acts and Sunnahs, or you believed all of them to be obligatory, or you believed all of them to be Sunnahs, or you believed an obligatory act to be a Sunnah or vice versa, or that every part is obligatory. This is supported by the Prophet's (peace be upon him) saying: "Pray as you have seen me pray." One should strive to gain a deep understanding of the religion, especially regarding individual obligations (fard al-ayn), based on Allah's saying: "Allah will raise those who have believed among you and those who were given knowledge, by degrees," and the Prophet's (peace be upon him) saying: "Whomever Allah wishes good for, He grants him understanding in the religion."