[Matrim]

Originally Posted by MarthX

It won't be directly explained but you do have enough info to piece together the answer.

Spoiler:

When Keiichi called Mion, asking her to invite Satoko to the festival, he was actually talking to Shion. Shion knew he was doing the same as Satoshi. She most likely told Mion and Mion had Teppei's body moved to a better location, just like she did for Rena in Tsumihoroboshi. Rena, Mion, and Rika pretended Keiichi was at the festival with them to create an alibi for him. Satoko thought her uncle was still alive because of Hinamizawa Syndrome. Rika explained that Satoko's disease can make her hallucinate and in the class room she believed the principal was her uncle.

Spoiler for no idea why we are putting spoiler tags but just in case:

That does sound logical except for the bit where Keiichi's friends failed to inform him they were lying in order to create an alibi for him. It just seems to me this was added to create a bigger mystery but it's so very implausible.

But still even moer importantly, we dont know WHY Rika is forced to relive this over-and-over

Because it's preferrable to just giving up and dying, don't you think?