The fullness of truth is possessed by the visible Church in full communion with the successor of Peter. The path of her sacraments, founded by Jesus Christ himself, is the only sure path to salvation. All humans should be invited to unite themselves with her. Others, not in full communion, such as schismatics (who since they seem to ignore the doctrines of papal supremacy and full governance of the Church of Vatican I cannot I think be said to be fully accepting of the Deposit of Faith, Scripture and Tradition), other separated Christian brothers, and even those further from the truth outside of Christianity, MAY (not will, but may) be invisibly united with the visible Church by following the law of God written in their conscience. The Deposit of Faith, Scripture and Tradition contain both the words of St. Paul (and Pio Nono on the possible salvation of the invinvibly ignorant) suggesting that there might be possible some sort of salvation outside the visible Church, as well as teaching of Unam Sanctam that you mentioned and the maxim "Extra ecclesiam nulla sanctus". My reconciliation of those teachings is the Catholic one. Your interpretation only accepts the latter, while ignoring the former, and tends towards Feeneyism, which has been condemned by the Magisterium.