At that point, it isn't entirely clear how much lee-way Kelemvor has. According to #12, it seems like he's trying to get them to the right place. The two big possibilities that jump out to me are that either a) anyone who makes it to him gets some sort of punishment, or b) he can punish or provide whatever other consequences he sees fit. This would allow for him shucking their souls of the Forgotten Realms dead-net and letting their souls float to an afterlife that accords to their alignment.
The last option (turning them loose from the Forgotten Realms dead-net) seems unlikely, because I'm having a hard time coming up with a scenario where Kelemvor would feel that he shouldn't do that. It is however possible that he might send them to the domain of whatever deity he feels they fit the best--giving them a third chance. Perhaps the ones he keeps around as servants are apathetic, while those who get turned into larvae are just generically evil (I mean, who really wants to sift through shades of black deciding which deity to send them to? Icky), and those who go into the Wall are those who are just standing there insulting the gods and refusing to accept assignment to the realms of any of them.
Of course, the problem with that, as I brought up before, is that it just seems like that number would be so small that the Wall would blow away if a demon breathed on it.
In any event, while 5e isn't entirely clear on what happens, it does flatly contradict some previous materials, and allows for quite a variety of interpretations within the text we've been given.