As I’ve said many times before, it is nearly *impossible* that not a single pope, theologian, canonist, nor saint caught this supposed error in the interpretation of Trent for 400+ years.
I don't follow. What "error" are you talking about? Trent didn't err. St Alphonsus would be the one that erred and he's not infallible, so not a major issue.
There's no "consistent interpretation" of Trent for 400 years....that's the whole point in showing that St Alphonsus' explanation is different from Trent.
What is extremely more likely is that a few laymen are misinterpreting the passage or misunderstanding that the “initial” justification, as you termed it, is only applicable to the actual sacrament of baptism, not to the graces that can be obtained from the desire of the sacrament.

No, we're saying the exact opposite.
1. The sacrament of baptism provides "initial justification".
2. If one desires the sacrament, then (according to BOD), they can obtain this "initial justification" prior to the reception of the sacrament.
3. You only get this "initial justification" one time. That's what the word "initial" means - first time only.
St Alphonsus' theory is that BOD provides a justification which is not 100% (i.e. that's why he says temporal punishment still remains). His theory is contrary to Trent, which says that justification ONLY comes THRU the sacrament. And the sacrament ONLY provides the "initial justification" which remits all punishment.
So either Trent is right, or St Alphonsus is right. Not both.