Beneath Stone and Sacrifice

I was a little disappointed with the finale to the Between Ink and Shadows series. I had such high hopes because it started off so intriguing, but by the time I got to this book, it wasn't holding my interest. I don't fully understand the situation surrounding Nim's father's contract and how that gave her access to magic that she didn't have to pay for in sacrifice. She has been hurt by magic before so she's not like Wes, yet I don't think she could use or summon magic on her own. The vagueness and unanswered questions regarding something important to the plot is an unfortunate problem that has popped up multiple times throughout the series. I wish everything was a bit more fleshed out and took its time to happen.
I also don't understand why Rhen was willing to be such an ally to Inara and become an enemy of the Trust. I feel like we didn't learn enough about him for his motivations to make sense. He didn't grow up with Warrick, so how is there that kind of sibling bond that makes them want to work together and help each other? I felt that Nim trusted him too easily and him being a good guy after being raised by the evil Head of the Trust with only Callum as an older brother/role model doesn't make sense.