Torn,Just look at the language used in these Teachings. It is easy to see that laver or desire is meant to be inclusive. There are other teachings from the past Popes and in Scripture that use the word "AUT" to mean "AND" even though it's translated as or. It means that the whole of it is included. (I can provide other examples from MHFM if you would like).When you get to Penance the meaning is clear as it uses terms like "at least", "either/or", "not to be ascribed...independently... " and even explicitly say "before the Sacrament be actually received". The Council goes out of it's way to show how Penance is received in desire. The differences between the Initial Justification and the Restoration are right there in the wording. If Justification could be obtained exactly as the Restoration could the Council would have been explicit and used words like "without (either) the laver of regeneration or (at least) the desire for it". The "either"/"at least" changes the meaning.
Simply ask yourself (or look into it if you don't know) if Trent indicates that perfect contrition with a desire for the sacrament of penance can justify? If it can, according to Trent, and the sacrament of penance is "necessary unto salvation; as baptism itself is," then the sacrament of baptism can also avail to justification with a desire for it and perfect contrition, just like the Catechism of Trent says, and which the Council indicates above.
Is the sacrament of penance "necessary" for those who have fallen after baptism? Yes. Yet the grace of that sacrament is available with perfect contrition and a desire to the sacrament, without destroying the sacrament's necessity.
Trent S 6, Ch 4: "And this translation, since the promulgation of the Gospel, cannot be effected, without the laver of regeneration, or the desire thereof
, as it is written; unless a man be born again of water and the Holy Ghost, he cannot enter into the Kingdom of God."
Trent S 6, Ch 14: " and that therein are included not only a cessation from sins, and a detestation thereof, or, a contrite and humble heart, but also the sacramental confession of the said sins,-at least in desire
, and to be made in its season,-and sacerdotal absolution; and likewise satisfaction by fasts, alms, prayers, and the other pious exercises of a spiritual life; not indeed for the eternal punishment,-which is, together with the guilt, remitted, either by the sacrament, or by the desire of the sacrament
,-but for the temporal punishment, which, as the sacred writings teach, is not always wholly remitted, as is done in baptism'
Trent S 14, Ch 4: "The Synod teaches moreover, that, although it sometimes happen that this contrition is perfect through charity, and reconciles man with God before this sacrament be actually received
, the said reconciliation, nevertheless, is not to be ascribed to that contrition, independently of the desire of the sacrament
which is included therein."