[Used Can]

Originally Posted by sky black swordman

What are the chances Saijō Yura wiil join and expand Eita's harem ?

Who knows? But from what I've read, she was most likely introduced as a support character for Chiwa. In fact, on the first link for spoilers I posted, someone pointed out that, by volume 6 all the girls but Masuzu had someone rooting for them:
Chiwa has Saijō
Himeka has Mana
Ai has the Public Morals Committee

As for some spoilers:

Spoiler for Volume 6:

These are the parts I have clear:
- The story restarts immediately after the point at which volume 5 ended - i.e. Himeka finding out about Eita and Masuzu's fake relationship. Apparently, (this is from volume 5) Masuzu didn't like the idea they were crossing the point "fake" relationship, so she and Eita started saying "Fake boyfriend" for about 10 times, which Himeka overheard.
- Himeka confronts them about it, being unsure herself about things since she seemed to think of Eita and Masuzu's as some kind of ideal relationship, so the whole "fake" deal confused her ideals on love.
- She really took a hit, since she realised most of the front those two people she admired was a lie, but she realised she was a liar herself too, since she wouldn't be honest about her chuunibyou either. So, she took the resolve about being honest about it at school and so she did. Surprisingly enough, people at school seemed to be cool with it, and even people who never paid attention to her before, became interested in her now. From what I understand, she was supported a lot by the Public Morals Committee, which seems to be full of bros.
- After that she confronted Eita and Masuzu once again. It seems that back in middle school, Himeka tried hard being popular but failed, afterwards she simply kept lying about herself, but now that people have accepted for who she is, she decided to stop lying about herself. So, now she can face Masuzu and told her that Masuzu was lying to herself.
- It seems that, after that, Eita took some resolve and asked Masuzu about the behaviour she's been displaying as of yet, what exactly did she think about him, but Masuzu denied everything. She said that as Anti-Romance, she hates love and that she's never though of Eita as anything but a fake boyfriend.
- Eita seems to have been convinced by Himeka's actions, and he's also ready to move forward from his insecurities about his past, and he gets ready for what may happen to him. At worst, the "contract" would end and the info in his notebook would be leaked.
- Despite that, in the end, he decided to compromise on the idea of not crossing the line (of fake lovers).
- There's a bit revolving about Eita's diary, regarding his resolve to cure Chiwa and the strength behind those emotions. This made Masuzu think back in the day she'd be a fearful opponent and that's why she decided to be vigilant of Chiwa and was hostile to her. Even so, Chiwa herself never noticed any of that. Masuzu though that if Chiwa was ever serious, she'd fall into anxiety. But even then, Masuzu still tried to deny her own feelings.
- Masuzu's father showed up at school and Eita felt worried about Masuzu, and decided to pay her a visit at her home. However, he caught a cold and Chiwa wouldn't let him do anything reckless. By the time he returned from the hospital, he found his notebook left in front of his house, and he received a mail from Masuzu saying their contract was cancelled.

Anything after that I'm not sure of.

Originally Posted by nfsac

Also I think Ai is the one who got most development and the only to fight with Masuzu, who victory is almost certain.

I don't think Ai has much of a chance in the LN. Even larethian, Kunagisa and some Japanese posts I've read seem to point out neither Himeka nor Ai have much of a chance. That said, Ai has her own spin-off manga, so she'll probably win there.
Chiwa, on the other hand, is the person closest to Eita, confessed clearly by the end of volume 4 and kissed him, and then kissed him again in volume 5.