I've just finished watching the first and second season. Awesome series! I have some thoughts about Locke's questions.
Spoilers imminent. Don't read if you haven't watched this!
1. How/why do Mion and Rena have the level 5 virus to infect Keiichi at the end of episode 4, season 1? And why do they stop going psycho once they inject him? The closest answer I can find is that Keiichi is already showing Level 5 symptoms due to his stress level and imagines everything that happens with Rena and Mion, including them injecting him with the virus.
I believe so too. Becoming paranoid and delusional is part of the Syndrome. Keiichi became paranoid and was delusional by then, and likely imagined everything about Rena and Mion. They never suddenly turn creepy and asked who he had been talking too. There never was any needle in the food. And they never injected him with anything. It's the same situation in Shion's arc when she believed that the Sonozaki House was involved in the yearly killings. Everything that the villagers said, or which Mion said which indicates they were responsible, may all have been her paranoid delusions. As we learn later on, the Sonozaki House was not connected to the killings at all.
2. Why does Rika have the virus at the end of the Shion-is-evil arc? She tries to inject Shion, but it never explains, in my mind, where she got the syringe. It's POSSIBLE that it isn't the virus in the syringe, but it seems implied that it is.
I think you got this totally wrong. Remember the time when Keiichi talked to Rika but did not want to admit he entered the forbidden shrine? That was when Rika talked about cats (Keiichi's group) entering the shrine and a dog (Oyashiro) was watching them? At the end of that conversation, Rika promised Keiichi she will do everything she can to protect Keiichi.
After that Shion (pretending to be Mion) confronted Rika and gave an order to Rika to kill the last shrine intruder (Keiichi). Now Rika knew it was Shion all along, since she's lived through it many times. To keep her promise to protect Keiichi, Rika decided to subdue Shion, whom Rika knows is on a rampage. The needle Rika had was not the virus, but it was likely one of Dr Irie's virus-suppresion + sedation drugs, which Satoko had been using. When she failed and got hit with the sedative injection instead, she knew the crazy Shion would torture her, so Rika killed herself before Shion could.
That is my personal opinion anyway. I don't think Rika would ever try to kill anyone.
3. Was everything just REALLY lucky at the end? I mean, it seems like a lot of the choices made aren't made by the main characters, but happen on a whim of the side characters.
I think there are a lot of luck involved, especially the involvement of Shion + Kasai was pure luck. And also the fact that Satoko's traps worked against trained soldiers is a bit far-fetched. It wasn't clear what the kids' plan really was. My understanding is this: They will fake Rika's death and show that the villagers are still ok after 48 hours, and hopefully Takano will buy it, thinks her research was wrong, and stop her plan. Oiishi's main role was to fake Rika's death. There's a good chance this wouldn't fool Takano, so the kids will slip out of their house and hide in the Sonozaki dungeon. They will make their stand in the mountains which is connected to the dungeon, where Satoko will have laid all her traps. And the last backup plan is to have Tomitake hide in a hotel, and notify the Banken if things turn ugly. It is not clear who the Banken is loyal to, but they are definitely not sympathetic to Takano or her superior's cause. Having 2000 dead villagers would be troublesome politically for the Banken's superiors I am guessing.
4. Not a big question, but what happens to Keiichi at the end of the Satoko arc? He's the sole survivor. Later both Satoko's end and Rena's future are explained. Actually, in that same idea, why do Rena/Satoko/Keiichi, in separate worlds, survive the effects of Rika's death?
Good question. I have no answer.
Edit: After giving it some more thought, I've come up with a theory. In Rena's arc, she was suffering from level 5 terminal disease, she was scratching her neck. But after her fight with Keiichi on the roof, she overcame her terminal disease and regained her sanity. Essentially her disease was cured. So she survived and lived to become a depressed, withdrawn adult. Yay.
Now no one else overcame the disease. In the world where Satoko survived, she was killed the next day, it was indicated that a nurse was responsible for her assassination.
In the world where Keiichi survived, his future was not shown at all. I would presume he would've killed himself after 48 hours.
Edited, Oct 23rd 2009 10:12am by McGame