Basically. What “we know” is that by coming to the Abyss, Melissance and her people were cured of ghoulism.
Now, if they go back to the material planes, will the ghoulism return? Not clear.
But here’s the thing, if someone from an antimus plane shows up in the abyss, they can go back to their antimus plane and be antimus again.
No different than positive people.
Presumably from the Abyss, demons could go to either antimus or animus planes, via summons/incarnation (presumably not via portal but who knows) not sure if anyone has tested it though.
Now, the thing is, there is animus and antimus. What the Unlife, however, does is “consume” animus to fuel it’s life in an oppositely charged plane.
The life draining is not so much a function of the fact that the Unlife/Undead is negatively polarized, so much as it is polarized in the opposite direction from how the body was created. In order to keep the body from falling apart, the undead/unlife needs new animus to replenish what it is destroying.
As far as the other thoughts you espouse….
[SPOILER]:-$ We shall have to see. [/SPOILER]