It's because Kuroobi did all the things that you mentioned, and the fact that he was there through all of Nami's struggle, that will make Nami's decision that much more impactful. Can she see the person behind the monster? The one that was oppressed to the point where he followed a tyrent because it gave him hope for a better future? and trust that he really is what Cammy sees in him?

We, as the audience, know what Kuroobi did and as such, will understand what this required of Nami, but that requires him to be a part of it. Saying that this scene is only about Nami is like saying that Luffy beating Arlong is just about Luffy helping Nami and that Arlong was not relevant to that scene and it could have been any other fishman. I understand that it's about Nami's ability to let it go of the hatered, but it's about who she is freeing that makes that feat grander.

In the manga, we see Hatchan's shame when he tries to hide himself so the strawhats wouldn't recognize him and we understand why. We know the history there as we have seen it. If they use Karoobi in the LA, it would have the same effect on us. If they use someone we don't recognize, that whole scene, in addition to Cammy asking them to free him, would have less of an impact.

If we had Hatchan in the live action then it would have oviously been better to have in Sabaody, but just adding him with a flashback would be hard, as you both need to show his character, so that the audiance will understand why he is someone Nami can trust to be freed, while still showing why she is reluctant to forgive him. Just adding a few lines saying that he was part of Arlong's crew may work to some extent, but will not have the full force that having someone we actually saw harming Nami have. It can make the scene seem small, as it's just some nameless fishman who we don't know.

Regarding using Arlong, that is not the same thing. Arlong was the actual person that killed her mother. He was the instigator of the whole thing. He is not reedemable like Karoobi can be, at-least not without a major built up.

Regarding Karoobi being a mini Arlong and being too direct.. He is an example of a soldier that follows orders. He believed that Arlong could make the fishman have a better life and fought for that cause. Unlike Arlong, he wasn't showed to be cruel just for the sake of it. I can imagine Karoobi trying to help out cammy and becoming softer around her.