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?
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?
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.