@koroshiya
pls be aware that the requirements that you listed above are only a guidelines. that is most likely why you hear so many different stories on naturalisation. each application will be looked at individually. for someone who is not married to a japanese citizen ten years seem to be the period that is most often mentioned. the government is apparently investigating whether or nor to introduce a point-system for visa/pr/naturalisation. i know a couple of years ago MoJ went to UK to study its system.
current legislation on naturalisation
THE NATIONALITY LAW