The Sunstone was Ge’el’s idea and not Avallac’h’s. However, just like you I, too was expecting much to happen. Did you notice the moment when she says “I give you my Heart? What kind of spell is this?” she actually casts only half the spell. There is a sound and Avallac’h does something. Not sure if something really happened there.
BTW, I love how soft their voices become around those scenes and when she casts the spell (in the English version). Almost like whispers…
These are just some ideas, concluded by the so many subtle hints in-game and books. They could be on the right path or they could be totally wrong. The truth is only CDPR know where they intend to go with their story, that is if there is going to be another game with Ciri as the main protagonist.
About the Empress ending, it is Ciri who says to Geralt that although she does not like that option, it is the only one she has if she wants to be able to do something good! (What does she plan to do? )
There is yet another hint in that dialogue, when Geralt asks her whose idea it was for Ciri to become Empress. Ciri does not reveal whose idea it was but she says that Yennifer had nothing to do about it.
We know that Philippa wanted Ciri to become Empress so that she becomes her royal advisor but I don’t believe Ciri would ever permit Philippa become her mentor. In short, since it is not Geralt, it’s not Yen, Philippa and Emhyr are out of the question, the only other person who could influence Ciri at such a level of decision is Avallac’h himself.