LARobert

Firstly the Catholic religious community Slaves of the Immaculate Heart of Mary is recognized by the Catholic Church. They have received canonical status in the diocese of Worcester, USA:
Nowhere in the Letter of the Holy Office which you have quoted does it say that Fr.Leonard Feeney was excommunicated for heresy. He was excommunicated for disobedience. He did not go to Rome when called to defend himself.The excommunication was lifted without him having to recant or make any changes in his writings. http://www.catholicism.org/downloads/Peter_Vere_SBC.pdf

The Slaves of the Immaculate Heart of Mary still hold the same beliefs as their founder Fr. Leonard Feeney. He taught that every one needs to be a visible member of the Catholic Church and there are no exceptions. For him the baptism of desire and being saved in invincible ignorance were not exceptions to the dogma.
Regarding the dogma Unam Sanctam, if you say that there were no real ex cathedra dogmas then which is ‘the dogma’, the ‘infallible statement’ mentioned in this Letter of the Holy Office 1949 which you have quoted here and which was issued during the pontificate of Pope Pius XII ?
http://catholicism.org/category/outside … -salvation

The Letter of the Holy Office also did not mention any defacto known baptism of desire or being saved in invincible ignorance so it was a criticism of the Archbishop of Boston who postulated that the baptism of desire and invincible ignorance are exceptions to the dogma implying that these cases are explicitly known and so are an exceptions.For him there was defacto known salvation outside the Catholic Church.