From my point of view, I can say that neither the style of the book nor the contents give the impression of yellow press. One of the authors, Martin Fritz, is a German journalist who lives in Tokio and works for a German public TV station, ARD, and they are usually supposed to be "serious" and to not invent things to make a story. (Of course there are many differences but to give you an idea: they have a reputation a bit like BBC.) They definitely have a reputation to lose if one of them tells nonsense. So I really suppose he should be creditable. The other author, Yoko Kobayashi, works as a free journalist in Tokio.
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Fritz and Kobayashi have heard from someone who has insider information about the monarchy: "It is not imaginable that the IHA decided alone, without the consent of the emperor and empress, to stop the trips [of the crown princess] abroad."
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I will below (see "The kunaicho") give more details concerning what Fritz and Kobayashi explain about the IHA and its loss of power in the last decades. Here I will only say that they state that while the late Showa-Tenno, Hirohito, never meddled with the dealings of the IHA and just concentrated on his own role, the present emperor and empress actively make use of the kunaicho as an instrument to realize their own ideas, and that they insist to be informed about all official matters that concern the members of the imperial family. The executives of the IHA can do or say nothing against the will of the emperor.
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But the executives of the IHA nowadays do not stay there for more than two or three years. It is not attractive for them, as Fritz and Kobayashi explain, because most ministries entertain contacts with some of the big japanese companies. And when the executives retire on pension which they usually do rather early they have a good chance to get a nice position in one of those companies… But the kunaicho does not have any contacts of that sort, so nobody wants to be there. An old ex-executive who has been working for the kunaicho nearly all his life complains: "Many are grateful if everything goes smoothly, if the traditions remain unchanged. Their mentality is: never be absent from your work, never be late and do nothing, be passive." They do not want to get into trouble, they do not want to take a risk, and so they do what the emperor and the empress tell them to. The prince has no significance for them. When his time of power will have come they will, in all probability, already be somewhere else…
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Several times Masako and Naruhito tried to initiate certain activities, informed the executives of their general ideas and asked for suggestions how they could be put into practice, and again and again, they were rejected or ignored. (Fritz and Kobayashi do not leave a doubt that although "technically" speaking, they were blocked by executives of the IHA this could never have been done without the consent or even the special request of the emperor and empress.)
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Toshio Yuasa, grand steward of the kunaicho, told the press that for the interest of the monarchy the Akishinos were desired to have their third child. And he added that this should be done quickly in order to not further increase the big gap (in age) already existing between this new child and its elder sisters. Fritz and Kobayashi comment that for an executive, even if he was the grand steward of the kunaicho, it would have been impertinent and by far exceeding his authority to bring public shame on the crown princess in such a way. And they say that as he nevertheless dared to act like this it is to be supposed that he was not speaking for himself but had been ordered to do so by a higher power…