Well, Kurugaya "just being there to help Kyousuke" is kind of interesting. I have to see Kurugaya's involvement in the other routes. Still, in the Refrain arc, Kyousuke mentions that he decided the team members already.
The anime said that they were all people Kyousuke chose that had a lingering regret. Let's not forget that Yuto Tonokawa (Kurugaya's route writer) supervised the anime scripts. Kurugaya did have her own regret, that she couldn't experience emotions. Perhaps Kurugaya's dual role is to help Kyousuke, but her route seems very "real" and true to herself. Kyousuke and the bros always seem somewhat aware of what the task is to help Riki, but maybe this coins the idea that they're know as omniscient as I thought in that regard, and they're simply going along with Riki's ideas in a way that will push him along and find his inner strength, and aren't aware of the heroine's problem. I don't know if the peculiarities of the characters are well-defined among all the routes, due to having 4 different writers for the VN with varying levels of character involvement, and widely different stories and magical involvement.
But yeah, I don't think Kyousuke was like "Alright, I need a strong, beautiful, independent woman to help in my plan to help my friend Riki. Nothing else matters. Ah, Kurugaya fits perfectly!"
I think the pretenses of each heroine having a regret were always how the Little Busters were selected. I think the willpower thing, Kurugaya being able to change her route, may have been due to her unsatisfactory with her regret being filled. Satisfying her inability to feel emotions created more regret, the regret of not being able to be with Riki. When you think about it, the world is already somewhat manipulated to the will and thoughts of the heroines, as pertinent plot events are only brought up each girl's individual arc. Kurugaya was more aware than the others and more "special", though, but then what about Komari and her acts in Refrain? I think Tonokawa's heroines were both playing with the rules with their willpower. This might've been something planned by Maeda to stir things up a bit, since I think Tonokawa follows the Maeda-esque format well with the supernatural stuff, I guess.
Actually, I think there's evidence in all the heroine routes that each one understood the world they were in, but they for whatever reason, only Kurugaya does this.
Damn, now I really want to reread the whole VN after opening up Refrain to get those screenshots.
edit spoilered everything ;x