This right here makes me think that Bella is just holding a grudge for the sake of it. If Milla have really wronged her in a serious matter I think she would have a different response, I don't even think that Bella thinks about Milla and their relationship.
Milia on other hand seems much more caught up with it, maybe out of guilt. But I like your take about Milla maybe going to far, there is potential with this idea, although I don't really think Mila would do that.
I would like if there is a way to redeem their relationship, not to they become friends but to end any bad blood
I think given that Bella is not a forgiving person it makes a lot of sense. Milla wronged Bella and for Bella is all that matter, it does not matter if later on Milla regretted it, with Bella you only get a chance. Milla defended Bella of a bullying that is her fault, after mounths being a bully herself. granted Milla have her reasons but I can see why Bella would not want to befriend her.
Now I think that hold a grudge for almost 10 years over middle school drama is reasonable ? Not really but it certainly makes sense to Bella do that
I agree with what you're saying here - there isn't necessarily an equal gravity to each person's opinion. Bella might have just viewed Mila as some sort of deadbeat already, there's nothing saying they were great friends before this. Bella's childish mockery of Mila shows she's not really that upset - after all, she's not worried that Nika is defending Mila that much. Bella sees Mila as beneath her and not worth her time, only mocking her for the rise she gets from Nika.
Mila on the other hand, doesn't want to be seen as a deadbeat, the reason she joined in the bullying was to get the attention off her after all. Maybe it's from this experience she stopped caring what other people think? Regardless, she really does worry about it, she's the one that thinks what she's done is so bad.
Regards keeping a grudge, some people are like that. You don't have to fix everyone's opinion. It's often easier with some people to drop the effort, put them in a box. That seems to be what Bella's done with Mila. She's not out to hurt her, she's not out to fix things, she's not going out of her way to reset their relations. Bella's got Bella things to do instead. The relationship's going to stay where they both parked it, until someone tries to get over that speed hump.
As far as repairing their relationship, I'm sure Ocean has plans. It is a story about healing, and it's already pre-empted by Nika wanting Vic and Bella to meet so Bella can help look after Vic. There's no way that Mila won't be around at times. Nika moving to a shared house with Mila, and there being no Bella interactions? I'm sure we'll see some fireworks, especially if you're on both their paths.
I guess we'll see where it all leads.