There’s a difference between a ger who states ab initio, they are converting al tenai that they’re not going to keep a particular mitzvah, and one who falls into nonobservance of mitzvot over time, according to the Achiezer I cited above.
But who says kissui harosh for a woman fulfills any mitzvah? Ii think the case for non-covering has been thoroughly explained, as there are those who rightfully believe that a married woman’s uncovered hair is not erva.