Is King David an argument for purgatory?

Here's a Catholic argument for Purgatory:

https://www.firstthings.com/article/2011/11/is-purgatory-biblical

Several problems:

i) I think there's a bait-n-switch. To my knowledge, the traditional Catholic rationale for purgatory isn't the need for postmortem sanctification. It's not about remedial punishment but retributive punishment for the guilt of temporal sin. In addition, there's a role for human merit. So the framework is forensic rather than therapeutic.  

ii) In the case of King David, the issue is complicated by the fact that he was abusing his power as head-of-state. He was wronging other people. To keep him from becoming a tyrant, God punishes him. At best, we could only extrapolate from that case to analogous examples.

It's inapplicable to purgatory because in purgatory you're no longer in a position to wrong or harm others. 

iii) It also depends on what we mean by punishment. In David's case, it's not that God meted out direct, positive punishment. Rather, God withdrew some of his protection from David so that David was now liable to palace intrigue and sedition from members of his own family. 

Again, though, that's hardly analogous to purgatory punishments. 

In the same article, Prof. Anderson says:

The final move is one in which these merits are potentially transferable to another person.

i) But that undercuts the need for penitents to personally suffer punishment to remit the guilt of temporal punishment.

ii) In my experience, Catholic apologists denounce penal substitution as a "legal fiction", but if the doctrine of purgatory includes transferrable merit, then that trades on the same principle.