I guess my issue with the MHFM here is a plausibility one, is it really likely that all those doctors and theologians of the Church held to a *heretical* position? I'd be more sympathetic if its "heretical" like in the kind of way that St Thomas Aquinas' view of the IC was heretical, needing clarification by a future dogmatic statement, but the Dimonds don't argue this way, instead they just argue that its heretical *now*. If St Alphonsus was alive today, I think its very likely that they'd call him "obstinate and bad willed" if he heard all their arguments and still thought BOD made sense.
I agree that this debate ultimately wouldn't go anywhere. Because its ultimately an issue of plausibility.
As far as Trent goes, I don't think Trent clearly teaches a BOD, but I do think Trent clearly *allows* for belief in BOD, subject to future church rulings on the website. There would be no point of having included that phrase if they thought BOD was a heresy no Catholic could licitly believe in.
Yes, as you know, despite not believing in BoD, that's also my big issue with the Dimonds, their assertion that any understanding of BoD is heretical. It's very clear to me that the Church has long allowed this opinion as not inherently incompatible with the dogma that the Sacraments are necessary for salvation. I think that the Dimonds' argument from "necessity" is a bit simplistic. Even if BoD is merely necessary by desire, it would still be necessary, acting as the instrumental cause of the justification operating through the desire. You can't desire Baptism without there being Baptism. I could see disagreeing with that, but it's an argument one can make without explicitly denying Church teaching. If it's wrong, we have to wait for the Church to condemn the position before we can declare it heretical. As soon as one starts making syllogisms against the explicit text of a defined dogma, you're a step removed from denying the actual dogma.
Also, while St. Robert Bellarmine and St. Alphonsus were clearly not infallible, as the Doctors disagreed with one another on not a few points, and held many opinions of lesser authority and speculation, to think that the Church would declare them Doctors of the Church when they were teaching open heresy is a bit much. Also, the 1917 Code of Canon Law permits hope in a BoD ... even though it clearly doesn't teach it (an argument for another day). Also, Cajetan believed in BoD. St. Pius V regularly ordered stuff removed from his works that he felt were against Catholic doctrine. In fact, he ordered that Cajetan's opinion regarding "vicarious Baptism of Desire" be expunged from Cajetan's work ... but not Baptism of Desire itself.
Their case that BoD is heretical flies in the face of a large amount of evidence to the contrary. Now, might it be heretical per se and the Church will one day condemn it? That's quite possible. But, as you said, until such an explicit condemnation, it's impossible to hold that the opinion is heretical in the strict sense, even though it's OK to argue that it's objectively heretical ... just as rejecting the Immaculate Conception was always objectively heretical even before its defintion.