It's like what Clarste said there. The point of the story is not the urban legend, it's about an outsider wanting to get into Gensoukyo. The urban legend itself is just a side effect, which plays a role of a usable plot point. Same thing with the popularity contest and all the religious stuff are just pretext, when the main focus is how to restore Kokoro's control of emotion.
What urban legend got in and who's using it, does not matter much. Even so, that doesn't mean popularity and urban legend can't be the focus of other characters' story.
And about the Mokou thing, there's an explanation for that actually. In Byakuren's story mode, she noticed that Mokou is unaware that she's holding an occult herself. So she didn't pick that urban legend, like Byakuren for example, it just came to her because of the Occult Ball. It shows that the occult power is something uncontrolable and unknown, which is the reason Byakuren gather them to seal it. The odd nature of the balls happens in other story arc too.
The Occult Ball power to "teleport" things are teased in Byakuren's and Miko's story and ending, and with Miko, shows that it can only connect between 2 worlds(thus it won't function in Senkai so it stays where Miko puts them, whereas it teleported the Buddha statue where Byakuren sealed it in her temple. Byakuren also speculates that it will also affect living beings.)
Go to Ichirin's ending, where she's transported to the Outside World, it shows the actual effect of the Occult Ball and how being in the Outside World affects her.
So even though the others may not do something direct about Sumireko, their part in the story helps us unveil the nature of this Occult Ball and, you know, make the story more complete instead of having to explain them all with just a few characters story arc. Not to mention those who knew about the balls true purpose refused to explain it clearly.
Of course, there are those like Koishi and Kokoro where their story is essentialy about themselves, the urban legend is simply something motivate it. I like their development in this game.