Agree about Bella, wouldnt fit her character at all and it would annoy her way too much to play a part in a charade.
I strongly doubt that Nami is more important than Nika, i think they are equal, otherwise Katie and Nojiko talking specifically about him is kinda nonsensical. I think the difference in there treatment (from Noji) is simply because shes more worried about Nika than about Nami.
And yeah i agree Nojiko as a independent player is very unlikely but i could imagine some sort of deal or agreement.
Like maybe Nojiko agreed to raise them - after whatever tragedy happened in the past - but in exchange demanded that the Zane's (and their dangerous/criminal activitiys) stay the fuck away from them until they are adults. It looks more like Nojiko is a former/retired member of the "Zane clan/mafia".
I took the exchange between Katie and Nojiko more like two people who trust/respect/know each other but have a strong disagreement than being enemys of some sort. I think Katie/the Zane's is/are thankfull towards Nojiko.
And yeah the bookclub was awesome imo, so much possbilitys