The following speculations are based on what I've seen through EP 1-3 and spoilers of EP4, plus the TIP given by kj1980-san
1. Regards to Keiichi's life threatening/shocking events such as Rena's "USODA!",the needle in the rice ball, hit and run, door bell event, Mion's attempt to inject him with something and such,
a.) All of them did happen.
b.) None of them actually happened, they were just Keiichi's hallucination.
c.) Some of them did happen while some didn't.
I would like to go with c.) here
It seems to me like every/most of events in the early part of story were real until a 'certain point', when Keiichi was driven to his severe paranoia state, that the 'reality; we observed (almost fromK1's perspective) began to fused with the hallucination.
2. Regards to that 'certain point'.
2.1) As hinted in the TIP, it was suggested that K1 might be suffered from some damages made on his cranial area--a factor driven him(who might already suffered from bipolar disorder) to his paranoia state.
If I remember correctly, these are the events which might had introduce the damages to him
a.) Club activites -- They do poke, hit, tortured him quite a lot. Those events were intended for comedy though.
b.) When K1 saw Rena with a big, big knife in her hand, he fell backward and made a loud THUD. -- Well, it does sound silly, but considering he just did a shocking discovery somewhat like 1-2 mins ago...
c.) The Hit and Run event - Although not succesful in hitting him directly, K1 did fell into the rice field and I don't think that was a factor. However, I believe the truck tried to hit him for real. Is it because the driver wanted to injure him in his cranial?
d.) EP4 event 2 - <<added>> Somebody mentioned that K1 was knocked out by the Green Man e.) EP4 event 2 - I haven't see this EP myself. Some says that Mion tried to inject something into K1 and he struggled agaisnt it. Maybe K1 was hit during his attempt to do so.
2.2) His personallity changed after receiving 'an injection'.
a.) The injection given by a doctor.
b.) The injection given by Mion (Which some speculated that it was merely a dream of K1)
From 2.1, d.) is going to be a crucial key for the final answer even though it weren't the one driven K1 to this state . About the injection, I think a.) sounds more plausible. Still, I'm going for 2.1.d)
3. What did Rena and Mion really try to do?
a.) Nothing, as every freakin events were rather K1's hallucination
b.) They tried to expose of Keiichi
c.) They were actually tried to helped him.
d.) They were trying to helped him but they/one of them changed their/her mind and pursued to kill him instead.
I would go for c.) and d.)
While watching, I had a feeling that Rena and Mion just tried to prevent K1 from becoming the second "Satoshi". The injection given by Mion might be a cure to what the doctor gave him. HOWEVER, I'm not saying none of them were responsible for the barabara murder. It's just...they/she didn't really want to kill K1 unless a situation kicks in.
4. Was Rena really a murderer?
a.) Yes.
b.) No
c.) We don't know for sure ^o^