Thank for the advice, I adjusted my strategy, and viola! sucess, now Lia is avalable.
(Only Rhean did not join my fleet)
So I am a bit annoyed that the Breanna battle did not trigger for me...

The thing with a sandbox game, is something like that can spoil the game a bit. You don't know if it's a bug or if you did something wrong. I re-did the Council meeting where I offer Rhea the necklace and offered her my assistance, but then nothing came from it (She just missing from the next meeting).
It's quite a long game to replay from the start again for just a small scene that's missed. But it is a pity, since it put's a negative on an otherwise awesome game. It's still possible that I did something wrong, but... who knows.
Thanks for you help. I eventually managed to get Lia by just adjusting my battle strategy a bit.