RTF is a global modification that replaces all towns in the game.

I see it this way: it's possible that Sareth will usurp the throne :) but he probably doesn't want to destroy everything - now he is a king and will rule his people.
And regarding the Order of the Void:
1) Lucretia is still alive.
2) In Heroes 6, Sandro found the Ring of Sin, moved to its dimension, retrieved the Mask of Deception, and the Ring's world was destroyed because the soul that created this world went to Asche after losing the mask. In Heroes 5, the Ring's world still exists, which means a new soul is holding its walls. Considering Sandro's phrase, "the ring is now useless, but I will continue to study it," I concluded that Sandro's spirit now lives in the ring and is waiting to be released.

The Overlord is vulnerable only to Railegh (remember the Heroes 6 campaign), so Sareth probably won't be able to overthrow the Overlord (he clearly doesn't have enough strength). Therefore, we can't say anything specific about the demons. If Sareth freed his father, then he could have done so under certain conditions, but it's unlikely that the Overlord would listen to him (well, this is just my opinion, but it seems very obvious to me).
As for the necromancers, that's an interesting guess - how does the ring have its own inner dimension in Heroes 5 if it was destroyed in Heroes 6? I think there's some truth to what you're saying.
It will be difficult to keep everything within the lore, and if we believe that unverified information, then we currently have three factions: the Order of the Void, the demons, and everyone else. By the way, frankly, I find it hard to believe that gnomes and the Dark faction can be combined within the lore - the gnomes would rather go underwater to Shalassa than fraternize with the Faceless Ones.