Well, firstly, the "highway to heaven" approach to unbaptized infants is exactly what's under discussion here, as Fr. Cantalamessa has proposed precisely that. He has not grounded the salvation of unbaptized infants in some desire of the parents, but solely upon the universal salvific will of God:
"The fate of children who are not baptized is no different from that of the Holy Innocents, which we celebrated just after Christmas. The reason is that God is love and 'wants all to be saved,' and Christ also died for them!"
Secondly, the decree says that the opinions being circulated about the eternal salvation of unbaptized infants such as to permit a delay in the baptism of the children, for reasons ostensibly liturgical or grounded in convenience, are "lacking solid foundation." This certainly includes an opinion such as that of Fr. Cantalamessa, which equates all unbaptized children with the Holy Innocents, and it is not only dealing with the matter of whether children should be baptized, but also with why they should be baptized quickly: because the opinions allowing them heaven although they be unbaptized lack solid foundation, and this for those born to Catholic parents!
As regards the passage in the Tridentine Decretum de peccato originali, it clearly rejects the opinion that infants "derive nothing of original sin from Adam which must be expiated by the laver of regeneration for the attainment of eternal life." Therefore, without the laver of regeneration (at least in desire), infants cannot attain eternal life, and anyone dying in original sin does not attain eternal life. The same can be drawn from cap. iv. of the Decretum de justificatione, as I pointed out, and likewise from canon 3 of the 16th Council of Carthage: "baptism, without which [infants] cannot enter into the kingdom of Heaven, which is life eternal." Innocent III testifies to the same thing.
The sacraments are absolutely necessary for salvation, in re aut saltem in voto. Fr. Cantalamessa is simply wrong to dismiss them as only "ordinary means" necessary only by a necessity of precept ("They are ordinarily necessary and people who can receive them and refuse are accountable before God.") His opinion is at variance with the CCC, which says that Baptism is the only means of salvation known to the Church, and that there is a great urgency to baptize little children.
"The Christians of Carthage have an excellent name for the sacraments, when they say that baptism is nothing else than 'salvation,' and the sacrament of the body of Christ nothing else than 'life.' Whence, however, was this derived, but from that primitive, as I suppose, and apostolic tradition, by which the Churches of Christ maintain it to be an inherent principle, that without baptism and partaking of the supper of the Lord it is impossible for any man to attain either to the kingdom of God or to salvation and everlasting life? So much also does Scripture testify, according to the words which we already quoted. For wherein does their opinion, who designate baptism by the term salvation, differ from what is written: 'He saved us by the washing of regeneration?' or from Peter's statement: 'The like figure where-unto even baptism doth also now save us?' And what else do they say who call the sacrament of the Lord's Supper life, than that which is written: 'I am the living bread which came down from heaven;' and 'The bread that I shall give is my flesh, for the life of the world;' and 'Except ye eat the flesh of the Son of man, and drink His blood, ye shall have no life in you?' If, therefore, as so many and such divine witnesses agree, neither salvation nor eternal life can be hoped for by any man without baptism and the Lord's body and blood, it is vain to promise these blessings to infants without them." (St. Augustine, On the Just Desserts and Forgiveness of Sins, and Infant Baptism, I, chap. 34)
Are you sure he has not? Does he not presume that those parents and others who are intensely concerned for the fate of children who died without baptism because they had no opportunity for baptism come under the "saltem de voto" clause, the baptism of desire?
This much I grant you--he could have expressed himself far better the first time and clarified better in his clarification. But you read the worst into his ambiguities and I read the best. He has not been as precise as he needs to be, which is evident precisely because some are putting the most negative construction possible on his words and others are putting the opposite construction on them.
And the same thing is happening with regard to the tradition. You and Hermannn and others are reading the tradition in one direction; JPII, Ratzinger/Benedict and Cantalmessa in another direction. The latter could be wrong and you could be right. But Cantalmessa has not been explicitly heretical. We are required by fraternal charity as mandated by St. Paul and St. John in the NT to seek to interpret what our fellow Catholics say with caritas, not to put the worst possible construction on it. That's why the ITC needs to take this up and the ordinary or even extraordinary magisterium needs to be explicit about it. I don't see why it could not have been left more undefined, but residual Jansenism puts pressure from one side and syncretistic liberalism from the other side, so I suppose it has to be taken up. JPII thought so, Ratzinger thought so. But until it is dealt with more definitively, I think some reticence with the label heretic would be the Christian thing to do.