Slavery, Part II

Given Pius IX's other shenanigans, I have little reason to doubt the British observer.  But I don't think anti-Catholicism among abolitionists excuses the Church's failure to support abolition.  It certainly explains the reluctance of Catholics to join forces with the Republicans.  As a descriptive matter though, I agree with your take on this.  The Church saw abolition as linked to liberalism, which it opposed (in part because of anti-Catholicism among liberals), and so it, for the most part, was hostile to abolition.  Sort of an "enemy of my enemy" kind of thing.  But that is, in my view, reprehensible.

If you need a non-web cite for the Holy Office instruction, it's quoted in Readings in Moral Theology No. 13, Change in Official Catholic Moral Teachings (Charles E. Curran ed. 2003) (Paulist Press).  See page 69.  I think it is also discussed in John Francis Maxwell's book, Slavery and the Catholic Church (1975).

Finally, I don't think Hayes's point is that the Church enslaves the faithful as a general matter, but rather that its support for slavery was without regard to whether the slaves in question were in fact Catholic.  In Catholic countries, as I understand it, most slaves were baptized.  But the Church did not, as a consequence of that baptism, require their emancipation.