How can David have experienced the beatific vision if Benedictus Deus says no one did before Christ?

That is a contestable point, and it turns on the interpretation of 1 Peter 3:18-20, where Jesus goes down to the prison and communicates with the beings there. There is a belief in Catholic history that the saints of the Old Testament, like Abraham and David, did not go to heaven when they died but went down to Hades and remained there in their soul existence until Jesus rose from the dead, at which point they were released and taken to heaven. That is a debatable interpretation of the passage, and after many years of study I have come to the conclusion that it is not the right one. The Church has not made this a dogmatic doctrine that every Catholic is required to believe; if it had, I would never venture in that direction. But because it has not, we have freedom to look at the passage again, and my commentary explains why that interpretation was made, why it is not good, and why there is a better interpretation that fits Peter's context. Both sides have to admit this is one of the most difficult passages in the whole Bible, as Peter himself says that some things are hard to interpret. So I hold that David, Abraham, and Noah did not have to wait another two thousand years in this prison, but have been in heaven in their soul existence since the day they died. You do not have to agree with me, and I am not the only one who holds this.