I think Nephthys in the dark space is the same Nephthys outside. It is just that she exists in 2 diffrent "worlds" the world of the mirror (the dark let's say temple) and the one in the actual timeline. Not sure though.
The one in the mirror is the real nephyths and the one outside is also real one but she's split in sense that the part that genuinely loves Seth is trapped inside while the one who cheated is outside. Sekhmet's mirror has powers like hators mirror. If u see a person in it it will trap u if u don't have genuine feelings for them (a part of u that is) n the one outside will be the brainwashed/in trance person whose deepest desires would be amplified making them do things they would never do. So nephyths uses that mirror n saw Osiris in it n since she didn't love him she got trapped and the one outside went to take his seed as her deepest desire was to have a child with Seth ( they weren't having any since that demented Shrek had robbed Seth's sperms )
As for horus he was gifted the mirror by hator as she also possesses one and he saw Seth in it but since he genuinely loves Seth he wasn't trapped in the nothingness. Instd his deepest desire was heightened making him rape Seth in his trance.
As for why nephyhis has the mirror is a spoiler we shall see in the future chapters concerimg the green gremlin
I knew it! Been saying all along, that BOTH Nephthys were probably real, and she just got split in two (since these 3D characters can, and do have, contradictory emotions for the same subject)
So did this Nephthys cheat on Seth before she got stuck? Or did Osiris trap her there and replace her with a realistic dummy?