From what I understand, his fiancee and mother both claimed to have seen no stretch marks, but have found red marks due to tape on his chest, the cell he was put in was supposedly suicide-proof, the resort that Kitami's ex-husband owns, where Taiji stayed, is allegedly visited by the higher members of the Saipan justice system, AND according to a (dead and unsuccessful) petition on change.org, his death certificate stating "suicide" was revoked.. I can't remember the places I got all the alleged info (mostly from this forum or any other kind of English source of his death, as well as the aforementioned petition) so I understand it's controversial, but I feel that that's about as much as we're gonna get.
Plus, he's not the only celeb whose official death was so blatantly BS'd (of course it must have happened to reguar people too, but we generally tend to know more about celebs), and while you could argue that the conspiracies are controversial, some still stand out with a large amount of evidence. Like Marilyn Monroe, for example. Do some research on her death, and you'll see she was murdered.
Another celeb whose death people are confident about was a murder is Brian Jones. I will admit I don't know so much the details on his death so I will not say with confidence that I think it was murder, but it's something that everyone who likes him as a musician seems to agree on, so there must be evidence and it's worth mentioning as an example.
The point of the examples is that people make mistakes or go corrupt. Yes, I cannot say for certain the cause of his death, and I am not questioning your knowledge on death investigation at all. But I am willing to believe that his death was murder and received an unjust "investigation" because even the people we trust to bring order are sinners themselves.