When was Mussolini threatened with excommunication?
One cannot be excommunicated from a church to which one is not acknowledged to belong.
Mussolini was presumably baptized as an infant and to my knowledge never formally renounced Catholicism. That in no way precludes his being an atheist, pagan or jedi.
Mussolini veered in and out of religiosity. According to his wife Rachele, he was mostly irreligious until the later years of his life. He was given a Catholic funeral in 1957, again according to Mrs. Mussolini.
Baptized Catholic, married in the Catholic Church, had his children baptized in the Catholic Church, outlawed contraception and divorce, made Catholicism the State Church of Italy, threatened with excommunication, given a Catholic burial albeit a belated one ... sounds pretty Catholic. The church itself certainly seemed to believe he was.