How did paradox work in Kurugaya's route?
The reason behind the paradox was only implied in Kurugaya's Route, but was made much more obvious in the anime.  Kurugaya wanted to keep the world intact forever, and to keep Rin and Riki from waking up, keeping the Little Busters! together.  However, this went against Kyousuke's plan of doing what he could to save them.  The time paradox and the snow were caused by the conflict between Kyousuke's willpower and Kurugaya's wish.  In the anime, it is shown that Kengo and Masato are much more exhausted as the days repeated, as well as Kyousuke altogether disappearing.  This implied that Kyousuke's will alone was not enough to beat Kurugaya's wish, and that the others had to help.