I basically agree with you on everything you've said:
Izumi is the one who's more complicated to understand
Spoiler (highlight to view)
She's obviously not in love with Jun, or else she wouldn't have rejected him so easily, with a smile on her face. She wasn't in love with him when they were in elementary school as well. And she's started changing her mind after she saw Nene being really close to Jun. I have a theory, after reading ch 7, that Izumi identifies Jun with her father. Her parents divorced when she was in middle school and what if this happened because her father had another woman? Izumi knows what it feels like to be loved and cared by someone who then easily leaves when he finds someone "more important". Jun loved her but after he was rejected he seemed already very close with someone else. She wasn't able to keep her father by her side so she's desperately trying to keep Jun with her. And it was the idea of losing his attentions that made her turn so sneaky, when at the beginning she was very strong and similar in character with Nene
Jun is quite pathetic and weak.
Spoiler (highlight to view)
It's not always right to be too good-hearted when a girl is nice to you or asks for your help. It was quite selfish of him to agree to go out with Nene, while saying those things like "I'll go out with you but I still like her, so I'll probably hurt you". He gave all the decisions to Nene, like that issue was indifferent to him. I'm quite sure he was attracted to Nene from the start, because of her personality, while he loved Izumi mostly for her beauty and nothing else. So if he really cared about Nene, he would have rejected her, or he would pay attention not to be too close to Izumi. He looked sharp at the beginning regarding Nene's problems but he's not able to see that Izumi is "hitting on him" and psichologically bullying his girlfriend. In the last chapters I got the feeling he doesn't love Izumi that much, but she'll always be perfect and flawless to him, so that's why he can't find any faults in her no matter what she does or who she hurts. If something happens (like in ch 7) he wants to be the only one to blame. He really has a serious case of "knight in shining armour" syndrome.
As for Nene
Spoiler (highlight to view)
she's fated to have a serious breakdown if she keeps up the act. I liked her character at the beginning, but her being tough and strong is partly just a fake, as much as it was for Izumi. She's the typical example of a girl who builds a barrier around her because she doesn't want to be hurt. Sometimes she's brave, like when he told Jun to choose her, sometimes she's really weak, especially regarding what Izumi does. The girl did some horrible things, like sneaking in Jun's bed, and she still says it's ok, nothing really important happened, with a forced smile that's impossible not to notice. It looks like she doesn't want to attack Izumi in front of Jun because she fears that then he won't believe her, her ruining Izumi's perfect image, and will leave her, and she's fine as long as he's with her,as long as he doesn't clearly end their relationship. But at the same time she hates seeing Izumi manipulating Jun, so she's hesitant to let him go.
She's really a nice girl but did something really stupid when she asked Jun to go out with her. She lacks self-confidence and some self-respect. I'm sure that if she didn't stick so close to Jun then Izumi would have been tired soon of her tricks. If she had waited a little more they could have started a relationship without Izumi interfering. I hope she's the one who gets the happy ending, even with another guy
My rant was even longer