"The question of 'actual proof' is a tricky one, because SGI/NST has spread it's version of Nichiren Buddhism, with an emphasis on 'actual proof'. This really means 'personal experience' and 'tangible benefit' in their terms. While this is something that most human beings would take as 'proof', it is not what Nichiren meant when he said 'actual proof'. To Nichiren, actual proof meant that the predictions in the Lotus Sutra where being actualized in his very life. Without Nichiren, there is no spreading of the daimoku in the first five-hundred years, there is no persecution of the preacher, no establishment of the object of worship (the gohonzon), no fulfillment of the prophesies that are made in the Lotus Sutra.
If the definition of 'actual proof' was validated by personal benefit, then one only has to look at Nichiren's life to see that he lived in extreme poverty, under very harsh conditions, and he gave the appearance of not being 'protected' as a votary of the Lotus Sutra. In fact, many disciples gave up their faith when Nichiren was banished to Sado, claiming that Nichiren himself could not find a peaceful life, so how could his followers expect to benefit?
TO this, Nichiren answered that the benefit is to escape the 'burning house' of this world and to end the cycle of birth-and-death as a deluded human. By definition, only slanderers of the Lotus Sutra are born in Mappo, so to take faith is so hard, since the consequences of slander are then suffered in the practice. However, this is the way of retribution of past slander against the Dharma. Nichiren himself said that his sufferings were because of past lives, and he rejoiced that he could actually resolve aeons of retribution in this single lifetime, thereby being assured of Buddhahood in the future. Nichiren makes this point over and over again in many authenticated writings. (Here is the danger in relying on forgeries like the Ongi Kuden and about 55 other well known 'gosho' that are not in Nichiren's hand and do not reflect the doctrine of the Lotus Sutra, but some other 'later medieval Tendai' doctrines of the 'hongaku shiso' craze that actually state the opposite of what Nichiren actually wrote.)
It' s obvious that there are 'miracle' stories in every religion. In fact, if this were a criteria of correct doctrine, then we'd have to abandon the Lotus Sutra and follow any number of spectacularly successful religions in the mundane world. However, this world is illusions, so what good is the manipulation of illusions? It's like a dream world. If a religion can turn the nightmare into a pleasant dream, has that solved the problem of falling to hell for slander, after you die?