Symmetry Diamond is a side story that explores what would happen if Oriko saw Walpurgisnacht instead of Kreimheld Gretchen.
This For Want of a Nail scenario results in Oriko and Kirika becoming full-on heroes instead of the Villain Protagonists that they were in the main Oriko manga. In fact, when they catch Kyubey trying to make a contract with Yuma towards the end, Oriko blasts the furball with her Oracle Ray and gives Yuma an encouraging speech to speak up if she wants things to change.
The conclusion, The Last Agate, basically has them preparing for a battle that they know the probably can't win against Walpurgis, but they intend to try anyway.
Symmetry Diamond also makes it clear that for poor Yuma, there are only two fates in timelines where she doesn't contract: Either she dies from her mother's abuse, or, as per what happened in Symmetry Diamond, she manages to escape, and lives happily with her grandparents.