Sending: Speak With Dead, but better?

[Brookshw]

Any idea how this connects/coexists with the River Styx?

Before arriving in Baator, lawful evil souls take a dive in the River Styx, which steals their memories and turns them into silt. The souls then emerge on the river's banks as near-mindless lemures.

A similar fate awaits souls in Hades (as larvas) and in the Abyss (as manes), which I would guess also involves the River Styx.

Anyway, that means these souls did have memories before the fiendish waters stole them.

It's not relevant, petitioners memories are stripped while the soul travels through the Astral on the way to Baator (or wherever), coming out of the river has no bearing.