I have a theory on Naamah & her role in the end game:
Ok so we all want to NOT have to pick a side in the end game but I feel Nergal is too consumed with hate for Qetesh to see much reason so I can only see this going 2 ways that allow for MC to keep both factions; either MC must defeat Nergal or Nergal is convinced to join Qetesh against a common enemy. In both scenarios I think Naamah can help.
Scenario One: Nergal must be defeated
So i think "Nergal's Champion" is a mortal avatar for Nergal to actually walk around the mortal world in & get involved directly with the affairs in his war against Qetesh. We know that the story of PQ is kinda ramping to a conclusion (& theres talk of PQ2 so I feel this is gonna be a 2 part story, at keast). In this first scenario, Nergal is too far gone with his desire for vengeance & finds out about MC's dealings with the Qeteshians. Deciding he is no longer needed, Nergal attacks MC in a boss fight (MC could be aided by Erevi & ED: Both care for MC & even if Erevi is too loyal to outright side with her lover I think ED can "convince" her). After killing/defeating Nergal the Tower will convert to serving Naamah as their patron goddess & she will be more willing to work with Qetesh. Naamah has already shown interest in being worshipped as a goddess so her plotting to overthrow her Lord isnt far fetched. She may be plotting to convert all Nergalites into Naamarites.
Scenario Two: Nergal & Qetesh Make A Truce
In this scenario Naamah works with MC (directly ir through ED, her clone) to convince Nergal that it's best to work with Qetesh than against her. If the Lord of Light is the true BBEG then perhaps MC can show Nergal this & he will reluctantly make a truce (Heck we can still have a boss fight with Nergal if he needs a little "calming down" first). Once the pact is made then ab uneasy alliance is formed & both sides plot the fall of the LoL. Which could be a great build up to the next game with an obvious end goal in mind. Naamah would be less helpful in this run, using everyone more as pawns in her own schemes.
The reason I believe Naamah is the key here is her desire to be worshipped & she has seemingly picked MC as her first follower. She may try to replace Nergal as goddess of the Underworld or at least gain enough favor to no longer be subservient. Her clone ED is an extension of her & would have similiar desires, if not then I could see her at least being more loyal to Naamah & her daddy over Nergal if push comes to shove. Erevi may be harder to justify, either she would side with MC out of love or she may need a little "convincing" from her daughter. And we know that deities that share similiar domains/powers can share gifts (such as Qetesh returning Oksana's powers after her brother, Ths LoL, took them away) so Naamah could replace Nergal & there would be little change to Erevi, ED & Familiar'a powers.
That's just a theory tho.