What mythic path are you on?Also depends on how you treat her before then if she'll give it back or not.

kill her and loot her corpse. at this point, just chalk it up to the bad programming. the "elder gods" screwed up, so galfrey has to die.

You need to romance her, otherwise she will not think, that you deserve the power hidden within, and try to keep the book away from you.

Her giving the book back on her own most likely depends on your alignment/mythic alignment, doubly so during the choices with her (and your deeds while dealing with the angels?). I got it back from her before she died and when she lived while NG and LG w/o romance. If you're on either true neutral, chaotic or evil, it'd make sense lorewise she wouldn't give it back as the good queen.

It's completely optional. You just need to make sure you save the stargazers instead of going after Mutasafen immediately when given the dialogue option in act 5.

Save the books not the stargazers.

Yes. But at Purula Fall (?), you will have to fight Musatafen and let the survivor die. Otherwise Musatafen will leave with crucial research project that you loot on his body...

You don't date Areelu at all,that would be really really weird for her to want that.

I'm going to say it - Nyrissa was a selfish b*tch. Areelu, on the other hand, was content with just living in peace with her daughter, albeit satisfying her curiosity with outlawed knowledge.

Plus, unknowingly setting the curious player up throughout the entire story to then optionally convince Areelu that perhaps there is a bit of her daughter within them during the ending was pretty cool. And giving Areelu, or at least the mother in her, a happy ending was pretty sweet - as opposed to forcing yourself into an abusive relationship with what felt like a random romance option.