The thing we know with absolute certainty is that Nichiren appointed Six Senior Priests and that Nikko was certainly not his sole heir. Most all the issues we debate have some bit of elbow room for interpretation because they are religious constructs. Nikko as sole heir is a horse of a different color because it involves an historical event. Historical events can be looked into. When Nikko as sole heir is seriously looked into, there is no debate. No professional historian argues for it anywhere. The facts speak so loudly that the claim is beyond the pale with no corroborative evidence at all to support Nichiren Shoshu’s claim.
There are three transfer documents (he sure must have been busy his last few days changing his mind). The authentic transfer document gives leadership of the sect to six Priests, Nikko included. We know this is the authentic transfer, because everyone acts exactly in accordance with that document. At no time does Nikko do the smallest thing in the role of sole leader. He does not lead the funeral but rather marches in the rear rank (we have two accounts of the funeral, one by Nikko himself). He is left only non-religious personal effects by Nichiren and complains bitterly about that later. And he does act along with everyone else as a group of six, filling the role of secretary to the group, not leader of the group. When the government interacts with the sect, they deal directly with Nissho, the eldest of the Senior Priests, not Nikko and never once does Nikko try to insert himself into these sorts of interactions, explaining that they have it wrong and he should be dealt with as the leader. A lot of things can be said about Nikko good and bad, but shyness was not a trait he was known for. When he leaves Minobu he writes how sad he is to go, but one of the two fake transfer documents says Fuji is where the Kaidan should go, so he would certainly have been happy to go there.
The fake transfer documents themselves are rampant with bloopers. They do not appear on the scene until years after Nikko’s death and are supposed copies of copies of copies to explain away why the handwriting does not match anyone present at the time they would have been written. One of the two was forged by Taisekiji but they were not coordinating their forgery project with their sister Temple, the Nikko-founded Honmonji Nishiyama, who was also busy forging one of their very own. In the Honmonji Temple’s fake, they have the chutzpah (nerve) to make a reference to a “Honmonji” Temple as the Kaidan Temple. And finally, if Nikko was really sole heir, it was a secret, because no one anywhere at that time makes even a passing reference to it. No one cares enough about Nichiren’s last order to discuss it when it was supposedly done. No one cares enough to mention Nikko as leader while he was on Minobu and no one cared enough to discuss what would have been a coup d'etat when he walks off the mountain and goes (sadly) to Fuji. NIKKO does not ever mention it. Not once, not even in passing that he at anytime was Nichiren’s sole heir . Toki Jonin does not ever mention it. The Abutsubo family takes no notice, Lady Koo does not notice. Shingo Kingo does not notice. Not even a vague reference exists by any person, from his closest followers to the government, to the neighbors. But what every one acts in accord with, is the real and only transfer document, the one appointing all six Priests.
This is very bad news for Nichiren Shoshu because their primary excuse for leaving Nichiren Buddhism and creating a weird and Tendai-like theology not actually found in the Gosho of Nichiren Shonin is that the “inner” and “real” doctrine of Nichiren Shonin is not found in his writings but rather in a secret "Mind to Mind" transmission (see: Tendai-Shu Blood lineage transmission theories) that starts with Nikko as Nichiren’s special receptor and sole heir , an event which has left no trace in the historical record and no serious person could believe really happened.