I also found I had missed on region to reclaim which made the final battle earlier extra hard as I was still under part of the curse. This time I had full access to my spells and was level 20 for the final fight. But this was mainly to see what would happen.
So in chapter 6 I killed Nyrissa. Not totally out of anger though. It was a mix of anger and sadness for what had happened to her. I wonder if this was the best option (short of romance) as the other two seemed pretty hollow and empty - Eldest of dust and trampled hope … or having to keep fighting the Jabberwocky and living and dying over and over. At least death brings her peace and maybe her soul would go to Pharma.
Anyhow what happened. After she died the LK was pissed. So I went to chapter 7 where the worlds merged. It was exactly the same as if I had asked Nyrissa to be my ally - only Nyrissa stayed dead and did not come back. So I had to do it without her.
I did the final battle with the LK and got the same dialogue MINUS the option to become immortal with Nyrissa and fight at her side in an eternal fight against the LK.
I then choose the line [ATTACK] Fight and finish the LK.
The response wasn't what I expected. Instead of fighting the LK said lets stop this pointless exercise (since he can't be killed permanently in this case). So that just removed the fight/attack option and back to the other two options - become his herald or tell him you would forget about him and he would forget about you - after restoring things to normal.
So same ending pretty much except Nyrissa is dead versus fighting him or stuck in a life/death cycle.
All said and done now I think this is my favorite ending as I think death, after all she did and her hollow life now, seems the most fitting. Plus I gave the smack down to the LK
Followed extremely closely with the ending in chapter 6 where Nyrissa gets what she wanted and becomes an Elder. She refused my offer to return her soul (briar) as well. So I wonder what kind of Elder she would be. To my mind not a very nice one considering her title. But who knows. Regardless she got her position, the LK got his ending, and I … I got my kingdom back.
So once I decide on my canon ending I will delete the others, back up my saves, and start my Ranger Tiefling