Actually, I don't think Reborn are beings brought back from the dead at all. It doesn't say anything about them being corpses. When Tavion says they have been "Reborn in the glory of the Dark Side," (or something to that extent) I interpret it as being similar to the Christian saying of being "Born again." They haven't literally been "born again," they've just turned over a new leaf, started anew spiritually per se.
This is the way I see it:
The Reborn are Force-less clones who used the power of the Valley of the Jedi to obtain their Force power. After all, they pretty much look alike and act alike. The only difference is that some have been trained differently (hence the different sabers in JA) and some have been training longer than others.
As for Shadowtroopers, I think they're select stormtroopers who were given Force crystals that allowed them to tap into the Force and use a lightsaber effectively. On one of the Doomgiver levels, one stormtrooper says to another "Give us that armor and we'd be just as good," suggesting to me that it's the armor with the crystal that gives Shadowtroopers their power.
As for Cultists, they're just power-hungry individuals. Tavion gives them that power by using the scepter to endow them with the power of the Force.