Buddhavacana - words of the Buddha.
Using the same bibliographic premise surely it could be said that the whole of Shakyamuni's doctrines aren't valid if they're not available in extant writings written by him? Whereas in fact they were recorded by his contemparies as is the case with writings of Nichiren Daishonin that you claim to be inauthentic.
Some people believe that Shakyamuni's words were recorded in writing immediately after his passing. I have never seen any compelling proof that this is so.
Rather, indications are that Buddhist teachings were not recorded in writing for centuries after the Buddha passed. Until that time, his teachings were passed orally.
To deal with the question whether something was an authentic teaching of the Buddha, the test is whether it conforms to other things that are widely accepted as having been taught by the Buddha.
For some Mahayanists, this has consciously been expanded to assert that all true teachings are the teachings of the Buddha, whether they actually were taught by the Buddha, attributed to the Buddha, or even if they came from another teacher.
The question of authentic Nichiren writings goes beyond merely the question of whether an original in Nichiren's hand, or the hand of one of his direct disciples, exists. It also goes beyond whether a text, if not in Nichiren's hand or the hand of one of his disciples, was cataloged immediately after he passed away. The fact is, there are probably still some Nichiren writings that have been sitting in some family's storage vault for centuries still waiting to be discovered. Even temples discover things in their storage from time to time that they didn't even know they had. This is certainly the case with mandala gohonzon inscribed by Nichiren that still pop up from time to time.
There are two images inscribed by Nichiren which did not come to light until the last century depicting the gods of the sun and moon visiting him. When they first became public, people denied their authenticity. Now people generally accept them as authentic.
The analysis of Nichiren's writings have evolved beyond proving the existence of originals, but has now turned to linguistic analysis, even using computer programs to analyze the writings. One of the standards applied was to look at the content and deny the authenticity of anything that even sniffed of Original Enlightenment thought. This approach has been drawn into question. Others criteria have hinged on issues related to the Three Great Secret Laws.
Then there is the practical approach I've heard from some priests - the authenticity does not matter - what matters is that the message is on point - basically an appeal to Buddhavacana applied to the genuine and attributed teachings of Nichiren.
Nichiren-vacana - that is the real question about authenticity. But some of the questionable documents say some extraordinary things that would radically and fundamentally alter what we think was the architecture of Nichiren's thought.
Jury is out. Until then, aint nothing to do but single mindedly seek enlightenment with yourself as the guide.
Being out there in the world thinking for yourself can be the scariest thing we do in life. But the way I see it, to be an evolved human being - to even begin approaching enlightenment - a path we must tread by ourselves - there aint no other way.