As to "Sith Rey". I don't think Disney has the stones to turn her full on dark side. Probably posessed by The Emperor. That actually sounds decent as it would be at least more interesting than a lame vision or dream. Maybe The Emperor posessed both Kylo and Rey and Kylo is able to fight it but Rey succumbs to it. Then Force Ghost Luke has to intervene and hell breaks loose. Who knows with Star Wars anymore Haha.
Agreed.
If I had to guess? I'd say Dark Rey is a clone of some sort.
Would explain her seeing so many versions of herself when she looked in the mirror in Last Jedi, why she's so good at everything and why she's so attracted to the dark side.
Who she's a clone of, I have no idea. Could be the emperor as you said. Could be Luke, I guess, although I think that's a bit corny.
Would remind me of the whole X23/Wolverine thing, how they had to make Wolverine's clone female to get it to be stable or whatever.