How come the Seiryuu warriros don't come back as spirits? The four passed on Suzaku warriors and some of the Genbu and Byakko warriors do, so why not Seiryuu?
It's probably for the same reason that they can't see Mt. Taikyoku. Their spirits were too rotten.
Then how come whenever sometime evil arose within the Universe of the Four Gods after Miaka and Yui finished their duties in the Book of the Universe of the Four Gods, like The Four Demon Gods and the fake Four God (the latter from Eikoden), (real) Seiryu will join (real) Suzaku, Byakko, and Genbu in those times?
Just because its senshi were rotten doesn't mean the deity itself was rotten.
How can Miaka summon Suzaku in the OVA? She slept with Taka, and if she sleeps with a guy, she's supposed to lose the right to summon the god.
Didn't Suzaku-Seikun say that he needed the power of their love to fully regain his powers? Perhaps the true power of their love wasn't unleashed or seen as true/full love until they had, as they say, become one?
This troper thinks that there was a mention that the Suzaku arc (the last two OVAs and Volume 14 - 18 of the manga) was labelled as a sort of test. A test to prove the depth and strength of Miaka and Taka's love for each other. Think of the things that happen, which all serves as a way to drive Taka and Miaka apart, from Kaho to Ren and especially when Tamahome supposedly returns and everyone is shocked. The fact that Miaka ultimately realized that she loved Taka and not Tamahome was probably a strong indication that her feelings for Taka were not because he was Tamahome's reincarnation but because she fell in love with him as the person that he is. The fact that she and Taka slept with each other was probably just one more physical manifestation of their love for each other.
In several religions, it's often said that committing suicide erases all your chances in getting reborn. If that is the case in Fushigi Yuugi as well, how come Chiriko gets reborn in the end? Or is this an exception, seeing as it was more a Heroic Sacrifice?
Just because this is a tenet of various (not all) Earth religions doesn't mean it's true in the universe of the book.
Yui's mentality throughout much of the series. The girl's clearly shown to have a level head on her shoulders compared to Miaka as she's not as oblivious to things and one of her school's smartest students. However for much of the show, she gets duped into Nakago's manipulations when facts that are clear as day are known to her from observations within the world of the book that don't lead her to question things such as time progressing slower in the book than in her reality, the priestesses for each god having to be virgins while she believed she was raped for much of the series, and knowing Miaka would be oblivious to these facts and cares for her as equally as Tamahome. So why put her close friend and the Suzaku Seven through hell and keeping herself latched on Nakago, knowing that the guy treats his fellow warriors as tools and isn't hesitant to kill whomever is a threat to his plans or is of no further use to him?
A mixture of post-traumatic stress and lacking of knowledge. Far as this troper remembers, Yui never knew about the virgin requirement to summon one of the Four Gods, so her thinking she had been raped would not lead to thinking she couldn't summon Seiryuu. And even just about to be raped can be very traumatizing, along with the fact that she was in a place she didn't know, and Nakago was the first person in the book to show her some form of kindness - even if said kindness involved killing her would-be rapists. She clung to him since he was nice to her, yeah, he was telling her how her friend betrayed her by not helping her, but Yui likely chose to believe it because then she had something to focus her hatred, feelings of vulnerability and similar onto something concrete.
Except Yui clearly knew about the difference in time between the book and the real world since she was whisked out of the book during the early part of the series and read about Miaka's times in the world of the book before being brought back into it with her and Miaka trading places. This would have at least tipped her off to at least know Miaka had nothing to directly do with her misfortunes since one day in the real world is equivalent to at least several weeks passing in the book.
Post-Traumatic Stress, plain and simple. It doesn't matter if she figured out the time differences between Book and Real World, she was putting blame on Miaka because that was just how she coped. She was suddenly pulled into the book again, was attacked by several men and thought she was gang-raped by them and even tried to kill herself because of it. The fact that Nakago then spent what was likely weeks repeatedly telling her that Miaka had abandoned her on purpose, basically manipulating her with his words, was just one more layer to it. Having a level-head does not mean that you cannot be irrationally emotional about things and act just as badly.
How can she have post-traumatic stress from not being traumatized? Yeah, she thinks she was raped because that's what she was told and she uses it as an excuse to just act like a real jerk to pretty much everybody, but you can't exactly give someone a psychiatric disorder by telling them "Oh, by the way, I know you have no physical injuries or memory of the event, your weird foreign clothes aren't damaged or missing, and nobody in this huge country where you're a famous legendary figure with a connection to its most revered god is ever awkward with you about it, but just so you know, you were brutally raped in the street by a bunch of guys. I totally killed them with my sweet laser powers so, y'know, it's whatever, I just figured you couldn't go the rest of your life without hearing about this horrible event that you would never have even noticed if I wasn't telling you about it right now."
Yui was incredibly hurt when she overheard that Miaka came back to be with Tamahome and not to look for her, though Miaka had said otherwise first. Her best friend since childhood days put 'some guy' over her. That betrayal was likely the main part that hurt, with the verbal manipulation from Nakago just doing its own part, with some of his lines being that Miaka never cared for Yui as much as she liked to believe, after all. A feeling of betrayal, coupled with manipluation, and maybe even some part of her liking to have a reason to take out her frustrations on others. (If recalling correctly, there was indication that Yui kept looking strong, despite being annoyed that her parents were barely around and expected her to be mature about things, as well as the pressure of doing well in school.)