Instrumental does not mean impossible without.
An instrumental cause is the means by which something is effected. The decree specified no alternatives to baptism as the instrumental cause.
Also, you still did not fully address the following. If this can be shown to be false, your logic could possibly be accepted.
The Latin for the text below has: "sine qua nulli unquam contigit iustificatio." In the Latin original, therefore, the phrase "without which" (or, in the Latin original, "sine qua", is a feminine pronoun meant to agree with a feminine noun) refers to the "faith" (a feminine noun in Latin) and not to "sacrament" (a neuter noun in Latin meant to agree with a neuter pronoun). If it was "sacrament" the Council Fathers wanted to highlight "without which no one is ever justified," they would have written "sine quo."
Looking forward to your response.
I'm afraid to disappoint you, but after looking into it, it turns out, they are right on the money, about this particular passage, anyway.
The Latin of this passage from Trent is literally saying that the instrumental cause of justification IS BAPTISM, but then go on to say no one is ever justified without FAITH.
First of all, this only means that Trent did not clearly assert the absolute necessity of the sacrament baptism IN THIS PASSAGE. They did in the Canons on Baptism, however, which is folly to deny, and they also stated as the instrumental the actual sacrament of baptism. They posited no other instrumental cause.
Chapter III on Justification: "But, though He died for all, yet do not all receive the benefit of His death, but those only unto whom the merit of His passion is communicated."
And baptism is constantly expounded as the cause of justification in the decrees of Trent. Again, nowhere is there ever offered any other positive affirmation that it can occur by any other means. In fact, Trent also indicates that Faith is inextricably bound up with baptism:
Trent, Justification, Chapter VII: "In Christ Jesus neither circuмcision, availeth anything, nor uncircuмcision, but faith which worketh by charity. This faith, catechumens beg of the Church - agreeably to a tradition of the apostles - previously to the sacrament of Baptism"
So before Baptism, they do not have this faith.
Furthermore, as you were so kind to point out:
Pope St. Zosimus, Epistle Tractatoria ad Orientalis Ecclesias, AD 418: "By His death that bond of death introduced into all of us by Adam and transmitted to every soul, that bond contracted by propagation is broken, in which
no one of our children is held not guilty until he is freed through baptism."
If they die without baptism, they die guilty of the bond of death transmitted to every soul. No Solemn Magisterial teaching has ever contradicted this fallible teaching of Pope St. Zosimus. Many saints have actually said words to this effect as well, but you reject them because the same saints also at other times taught that an unbaptized person could be saved by perfect contrition (which is impossible without charity, which is impossible without supernatural faith, the faith which gives eternal life, the faith which the catechumens beg prior to baptism).
Well now you have a pope saying it. And he was never contradicted by any dogmatic decree. Do you believe him?
Of course, as you mentioned the Blessed and Glorious Virgin Mary is excepted from this (it was not transmitted to her soul), even though the good pope St. Zosimus didn't seem to have her in mind, and fortunately for him he was not denying any dogmas with this teaching of the Ordinary (fallible) Magisterium, since the Immaculate Conception was not defined until around 1400 years later.