Were David's future sins forgiven at the moment of his conversion? (Quickie Apologetics)

I'm not sure if I have posted this before, but I want to make a quick post about it. I'd say that 'moderate/intermediate' level of Catholic apologetics knows that when Romans 4:6-8 speaks of the justification of David in Psalm 32, that this prayer in Psalm 32 was not the first time that David came to faith. Instead, David had been converted to God since David was a young man (1 Sam 17:33-37). In this case of Psalm 32, David was praying about repenting of his adultery/murder in 2 Samuel 12:13-14, where as an adult David committed mortal sin and needed to repent. Thus, if Psalm 32 is talking about Justification, as Paul says it is, this can only mean David lost his salvation by mortal sin and regained it when he repented. This refutes/undermines the standard Protestant claim that Justification cannot be lost by our sin (or regained by Repenting). This brilliant insight was first made by Robert Sungenis about 25 years ago in his published book Not By Faith Alone.

That said, certain Protestants like James White insist that David's future sins were (also) forgiven per David's words of Psalm 32, which is a serious presumption since the Bible only ever talks of past sins being forgiven. That's because the Reformed are forced to teach all future sins are forgiven in order to uphold their other erroneous views, namely Faith Alone and Imputation (discussed many times on this blog). But what if we can look even further into David's life, years later as King, and see him falling into sin again? That would obviously cause serious problems to the White/Reformed thesis. And indeed there is such a text, discovered by the Catholic blogger [HERE], where he points out that the final chapter of 2 Samuel, specifically 2 Sam 24:10, speaks of an elderly David disobeying God in another serious manner:

10 But David's heart struck him after he had numbered the people. And David said to the Lord, “I have sinned greatly in what I have done. But now, O Lord, please take away the iniquity of your servant, for I have done very foolishly.” . . . 17 Then David spoke to the Lord when he saw the angel who was striking the people, and said, “Behold, I have sinned, and I have done wickedly. But these sheep, what have they done? Please let your hand be against me and against my father's house.”

It is clear that David had sin/iniquity on his conscience before God for his bad behavior, and David was again praying for forgiveness. It is clear that God was even punishing David for his sins and had to do Penance to fix it (2 Sam 24:24-25). This is impossible if David's future sins were forgiven years earlier when David prayed Psalm 32. This is impossible if David was "covered by Christ's imputed righteousness" such that God doesn't see David's behavior but rather only sees David as righteous at all times. I think this is a wonderful find and believe it raises a Catholic to 'advanced/expert' level when he includes 2 Samuel 24:10 along with pointing out that Psalm 32 was about David's sin in 2 Samuel 12. We simply must make use of powerful arguments like these, because they can be very effective against Protestants.