Sounds like you are making excuses to forgive her. MC's dad is an asshole for what he did, his mother too but the game was absolutely clear about that they did not force Kaija for anything. They tasked her to protect MC and that was it, how she was going to do was up to her.
If you are talking about keeping the order secret, it's not a lie from her part. She does not need to tell anything to MC if they were just friends. She chose to be that close, she chose to lie. Iirc, even their friendship was fading when they started to grow up. MC mentioned she was not the same person as before and he only catch glimpses of her old self time to time in her eyes.
IMO that sounded like they were not even going to stay as a friend, if she was not tied to MC by her task. Love path or not, she used MC's feeling. First their friendship, then the love. Now, this will be my speculation but I think she chose that path because she noticed they were not going to stay close friend much longer, so she jumped into relationship.
I measure sincerity of an apology through how willing they are not to repeat same mistake. Not gonna talk about evil bitch path since she is not even apologetic in that path but I am thinking she would do all of it, all over again if she ordered by Templars. Why do I think like that? Try to make her stay out of the fight when they stumbled into Elven Hellspawn skirmish, you'll see. So I really don't think she is sincere. Also not knowing other LIs is really not related to Kaija being redeemable or not.
You are right though, these stuff depends on personality and how gullible you are or how big your heart is.
I do agree being Fiona is the most interesting. Though, with last update Medb climbed the top for me. Especially when queen mentioned Medb's soft spot for humans. AFAIK, next update will be elven one so unfortunately we are not gonna see Fiona or Elea. Luckily no Kaija too.
Jokes aside I think, at least for me, the most crucial deciding point for Kaija will be the revelation of what happened to her parents and her grandparents. Whatever happened effected her deeply and changed her. I think she is justifying everything she does by that scar whatever it is.
Also, Kaija path being your favourite and Kaija being your favourite is very different things. I will always keep a save for her because she is crucially important to story and enjoying her path is a given, if you enjoy this game but forgiving such a person then putting her above all others is weird at best at this point.