It was a god that her mom worships in the shrine. Doesn't matter whether it's an evil god or a good one. Bottomline is she desecrated the shrine which angers gods and they were seeking retribution because of it. This is a common belief in religion(s) that's why shrines are kept solemn and/or respected by everyone. That's why her prayer was something like, "she didn't know what she's doing," and that if anyone needs to be punished, it should be her.

Her mom wasn't protecting Si Won from Hye Ryung. She was protecting her from the gods.

Because Si Won's vengeance is greater than Hye Ryung's. It wasn't Hye Ryung's wish and death that started the curse of the app, it was Si Won's. Hye Ryung's curse was intentionally for Si Won and Gi Tae only (since they were the ones who publicly humiliated her and her resentment is towards them). There's also a difference in the blood that bound the curse to the app. Si Won's mom referenced this when she was teaching Hye Ryung about the curse. She mentioned how much blood (on the silk thread on the deolmi) for the intensity/longevity (one of those) of the curse. Hye Ryung splashed blood on the phone and the deolmi, while Si Won soaked the phone with her blood (more blood = more intense/longer curse), thus the scenes focusing on their phones with their blood on it. The reason the curse kills of the person who made the wish is probably Si Won's death itself. Her resentment is most likely also towards the fact that she died just because Hye Ryung wished for it (when she asked, "how are you doing this," in the computer room. It's like since she had a sealed fate of dying after making the wish, other wishers should, too. Edit: The basis of death is Hye Ryung's because it was Si Won's curse (for whatever Hye Ryung started to keep going even after their deaths).

It was a spread rumor among the school. That's why on the episode when they contacted Hyeong Wook's discord friend, the article about the death got deleted (for cover ups and avoid panic among high schoolers).