So Migne seems to be walking it back with that latest footnote. Of course, there are very likely multiple editors involved, which would explain the inconsistency. Might be nice to see that second footnote in Latin.
You're right, EVERYONE who later accepted BoD hanged his proverbial hat on "the authority of Augustine and Ambrose".
Augustine floated the idea very tentatively, saying he went back and forth on it, ending with "I find ..." [in favor of BoD]. No sense whatsoever that this is received Tradition or that he's teaching it with authority, just very tentatively opining in favor of it. So this is the "authority" of Augustine, an admitted speculation that he had gone back and forth on? And then it's very clear that he retracted the opinion forcefully, issuing some of the most anti-BoD statements in existence. Even Karl "Anonymous Christian" Rahner had to concede that he later rejected the idea. So much for the "authority" of St. Augustine.
St. Ambrose rejects BoD numerous times, and if you look at the context of Valentinian, he says that he hopes that Valentinian could be like the martyrs, but then says that even those are "washed but not crowned". That refers to some remission of punishment due to sin, but without crowning, which is entry into the Kingdom, or Beatific Vision.
So BoD falls like a house of cards.
Not to mention that there's zero need for it. To say otherwise would be to constrain God's Providence by "necessity", as if God could be thwarted from getting the Sacrament to His elect (something that St. Augustine pointed out could not be denied "if you wish to be Catholic").