Let's translate the entire dialogue first
I'm going to go ask. If he's the culprit, then—
You're going to fight Arin?
Depending on the situation, yeah.
Arin's really nice, you know?
I see. That's exactly why I'm saying it's depending on the situation.
Please note that the translation of that last sentence is by no means a literal translation—the grammar definitely does not match word for word. First, だからこそ can also be used in the beginning of a sentence to mean "That's why". "That" refers to "Arin is nice". From your post, I think you understand this.
The mentioned situation exists as well.
This という is not quoting だからこそ. I feel like it is easier to look at という場合 as one word. The という場合(the said circumstance) is referring to what he said before「アーリンちゃんと戦う」.
So maybe to make this simpler:
アーリンちゃんと戦う ＝ という場合
And then if you put it into the sentence:
That's exactly why I said that there is a possibility that (I will fight with Arin / the aforementioned situation might occur).
Okay, then why would "Arin being nice" lead to "I want to fight him/her"? Well, the speaker isn't saying because Arin is nice, he wants to fight her. He's saying because Arin is nice, depending on the situation he might not have to fight her.
So maybe a more clear/literal translation would be:
Because Arin is nice, the situation where I fight Arin also/might exist.
場合もある→That possibility also exists.
This "also" emphasizes the possibility that the situation might not occur. The situation where they don't fight is the assumption from "Arin being nice". What he is saying that the situation where they do fight exists in addition to the larger possibility that they don't. He is giving Arin the benefit of the doubt, but expressing that there is also a possibility that Arin was the one who did it and they will have to fight.
I hope this makes sense. Please note that I am not a native, so if I made any mistakes, feel free to correct me.