[Joly]

Spoiler for Several things I wanted to point out regarding the Hinamizawa Syndrome...:

The Queen Carrier Theory

In the prologue of the Minagoroshi-hen VN, the reader is asked if they know something that happens in Watanagashi-hen and Meakashi-hen that Rika doesn't know. What is it? In those arcs, Rika is killed either by being tortured to death by Shion or committing suicide. It's assumed that "the terminal symptoms will start showing within 24 hours at the earliest, and 48 hours at the latest." Obviously, Rika doesn't know what happens after her death.

Events take place after 48 hours and the villagers do not fall into madness after Rika's death. The Queen Carrier Theory is proven to be false.

Are Mion and Rika susceptible to the Hinamizawa Syndrome?

Even if Mion and Rika are not shown going berserk in the series, this does not mean they aren't susceptible to the disease. Like others have said, there needs to be something to trigger the level advancement. Mion just hasn't received the proper trigger.

* MION

Directly quoting this from Rika in the VN, "Mion never lost her mind in any of the worlds... I guess she doesn't have the guts for that... Oh, I suppose that was mean."

* RIKA

These are mostly my own thoughts. Even if Rika is the Queen Carrier, I believe she can also be elevated into the state of L5 based on some inferences I've made.

Minagoroshi-hen

Another VN quote from Rika: "There were times when I went on a rampage too. However, I was killed instead... how pathetic." She possibly went into L5 in other worlds. Before this, she also describes the other characters as "getting into a rage", "lost her mind", etc. She doesn't specifically say they go into L5.

Conversation between Irie, Takano, and Tomitake in the VN. I believe this is shown in the manga too. This happens in Minagoroshi-hen after Rika warns Tomitake and Takano at the shrine.
"I want to believe that it's only her paranoia... She said there's no absolute proof either..."
"Rika-chan's test result from last week... I thought they came out pretty good, but did you see anything different, Dr. Irie?"
"No... It was stable as always."

"Why does she think that someone is trying to kill her? If she's experiencing paranoia, she may be in the early L5 stage..."

Not only does the clinic give frequent check ups for Satoko, but Rika as well to see if she does enter the Syndrome's stages.

Rika's thoughts while visiting Irie. This occurs after the festival and speaking with Oishi in Minagoroshi-hen.
"Perhaps I have Hinamizawa Syndrome, and this is a hallucination..."

Although Rika is not in L5 when these thoughts and conversations take place, it does seem like it is possible that she can enter those stages with the way the characters are fretting over it.

Watanagashi-hen/Meakashi-hen

"Do not use C120 if your test results are not positive.

If used on a healthy body, the symptoms of high fever, rashes, dilated pupils, and hallucination will be seen within 10 minutes." (These notes are from the end of Minagoroshi-hen. Apply this information to what happened in Watanagashi-hen/Meakashi-hen)

In these two arcs, Rika tries to inject Shion with the C120 medication. Instead, Shion takes the syringe and injects it into Rika, who is healthy. This is the reason why she becomes very weak. In the VN and the manga, she is shown/described scratching her throat out. The notes on C120 do not describe throat scratching as a part of the symptoms on a healthy body, but it perhaps triggered Rika's body to elevate into L5 Hinamizawa Syndrome.