Then we know that the watanagashi-hen is repeated hundreds, if not thousands of times, tso it makes sense that in the vn watanagashi, which isbased off the first repetitions she gets tortured to death, while in the anime watanagashi, she decides to better kill hersefl as seen in the meakashi (since meakashi and watanagashi are the same events from rika's pov).

That's the only arc that shows some crazy-completely wrong delusions, like the killer (during the act of killing) imagining another persn is doing the killings).

Onisarashi-hen, though similar to the other arcs in that it fosters intense delusions, is the major exception to the entire series. The feelings and situations created were the direct result of mental instability, since she had never been to Hinamizawa, the only place where the parasites are found. Since the parasites cannot (as far as we know) spread from person to person, Hinamizawa Syndrome cannot explain her behavior. Furthermore, there is not a single mention/depiction in Onisarashi-hen where she displays physical symptoms (i.e. throat scratching). The arc played more toward the "demon blood" explanation, pointing to perhaps a genetic issue and not an environmental one. The entire arc should not be considered in any explanation relating to Hinamizawa syndrome.

The needle contains C120, a suppressant for the Hinamizawa Syndrome. This is the same medicine that you see Satoko taking. Rika wanted to help Shion and was trying to give it to her by force.

Rika kills herself after being injected by the needle because everything was beginning to look hopeless for her, plus she'd rather commit suicide than suffer days of torture from Shion.

In Watanagashi-hen, she does not commit suicide (but the anime shows her suicide scene anyway. I'm guessing this is a mistake or a "flash-forward" to Meakashi-hen.) and is tortured to death. She wanted to avoid this fate in Meakashi-hen.

I going to have to doubt the "never been to Hinamizawa" part... considering that Natsumi did live in Okinomiya, and as the granddaughter of the head of the village council, would have at least attended one Watanagashi.
I'm also doubting the "the parasites cannot spread from person to person"- Keiichi of Onikakushi-hen is a prime example of this- Why would Keiichi go crazy in this arc if it weren't for the move?
While I can agree on the no-show of the physical symptoms, this was later shown in the 'bad-end version' found in Higurashi Kizuna 1.

What was Rena doing getting so close to Keiichi like that, when he pushed her? That seemed a bit OOC, even for her

She had her machete with her, to go look for cute things, or was she trying to "teach" him a lesson?

Take out the creepy camera angles, eyes, and tone of voice. Rena was just genuinely concerned for Keiichi (according to what I've read). Getting close to him wasn't nearly as threatening as it was shown to be, apparently.

And I've also read that Rena was trying to teach Keiichi a lesson, as he was carrying around a bat and people found it odd and a bit frightening I think. So Rena did the same, to show him that it wasn't really great to do that and that he should stop.

She wanted to protect Keiichi from whatever he was afraid of. She was trying to calm Keiichi down with her words because he looked so afraid, but Keiichi was delusional he sees and hears everything menacingly.
That is also reason why he saw the marker as a needle.

"That is also reason why he saw the marker as a needle." - when did they show it was a marker? i thought it may be Rena mistakenly put in the needle?

In the last episode of the first arc (Onikakushi-hen), you know Mion and Rena come to Keichii's house. And they want to punish him for their last game (guessing which ohagi was made by them). Then Rena grabs him and Mion pulls out a syringe. That syringe was actually a marker, but Keichii being in that desillusional state didn't see that, and so he thought they want to kill him. And then he killed them. The thing is they only wanted to draw a funny face on him. It shows this in an episode, close to the end of the first season.

And about the needle scene, you must be meaning the ohagi with a needle in it scene. Well, there was no needle, he just thought there was one, again because of his abnormal state.

It really was a marker. Rena and Mion wanted to write on his shirt just like they did to Tomitake during the Watanagashi festival as a penalty game. That's why Mion said that they were going to give him "the same treatment as Tomitake". And beside, Mion was trying to reach his chest with the "syringe", not his arm.