That sounds great to me! :D 

Edit: God this is a mega text wall/brainfart I'm so sorry XD

As to the other two points I think we touched on them but never ironed it out? 
-I like the idea of it being some sort of (remote, perhaps?) facility on Zakuul  that'll make it pretty easy for Vaylin and Arcann to call up a buncha those Zakuulan Knights to back them up and leave our heroes overwhelmed enough for Meetra to get axed :K 

As for Valkorion I beliiieeeveee I have a solution for that so hear me out here, cause I promise there's method in my madness ;) XD 
-So as far as I glean from the lore (and sprinkling in a bit of personal headcanon/embellishing) Vitiate is essentially an ancient Sith being/spirit who is so well-versed in the Force and using it to drain the life essences of others that he essentially cherry-picks bodies to use as avatars, and when one outlives its use and/or is killed (as with Voldon) his spirit then retreats back into the Force-void or wherever Force sensitive guys go when they die, till he finds a new host and so on and so forth

- I REALLY REALLY want to nix the whole SoR/crazy Revan brought Vitiate's spirit back to life just to kill him himself thing because it just spits on Revan's entire character arc from KOTOR and I DESPISE it XD 

-The Sith Warrior (to put it into perspective of "our" characters, one of the major changes I would make would be to make Vano the Warrior/Wrath, I can fill in the holes etc as and when we go back to that part but I think I can hack in some retcons for now to make it work ;)) storyline ends with the Warrior becoming a person known as The Emperor's Wrath - essentially they hold the same political / social power and sway as a Dark Councillor, and they're [i]technically[/i] "answerable" only to the Emperor. 

-Its then heavily implied throughout the later game content of KOTFE and KOTET that the "Outlander" (for sake of continuity let's say this is the Wrath (ie. Vano) that Vitiate possesses them like he tried to do with the JK (I think, haven't played that far into the Knight storyline yet oop? XD) but this time is more successful due to having a deeper Force connection with them (having been his Wrath this is plausible? cause story-wise it makes the most sense, they have the most direct connection to the Emperor XD) and he essentially almost makes their mind his "summer house"

-Vitiate then intends to regain his strength enough that he may use the Outlanders body as his new avatar (again assuming this is Vano/the Wrath, Vitiate would know that they have the strength and power he desired to be able to subjugate the galaxy and blah blah)

-It would give him a "door in" as it were, I dont know how we would then go about introducing him and so forth but I was kind of intending to at least shoehorn it in a little bit that Vitiate was still messing with Vano's head so I tried to drop little bitty hints here and there that Vano's not 100% right mwahaha. And being the Lorekeeper and all she could inadvertently give Vitiate a lot of info on our heroes and their movements and so forth  :K

-IIRC; KOTET ends with the Outlander and co finding an equally ancient Sith holocron that contained Vitiate's father (???) And persuaded him to help them Force walk into the Force-void place and kill him once and for all. 

We'd have to bridge the gap between them realising Vitiate was still a problem and dealing with both him AND his kids but...its not a totally impossible thing to work out? XD iirc : Vitiate actually "helped" them defeat Vaylin and Arcann in order to get them the heck off his throne so he could do the possess-y come back to life thing with the Outlander's body once they were gone o.o

Let me know what ya think and any (or all? Hahahaha) parts you'd want to tweak around