For what Petunia posted about the Base64 story, it could either be Monika or Yuri.
Though, remember in the story of DDLC, Yuri dies in Act (3?) and continually stabs herself. It would've made sense about the distorted story. Monika is actually a CHARACTER in the story, it has been running on a broken script, and she deletes herself from the game (blah blah...). So, isn't it Monika who wrote/spoke in that story? It would've made sense saying: "...Everyone is dead." Sayori is dead, Natsuki dead, Yuri dead. But? Monika didn't die, she just deleted herself. So, it would've made sense that Monika is the author of that little short story. (cough)