Only Namu Myoho renge kyo is correct because Namu Myoho renge kyo is the personal name of the Supreme Law. Neither Nam Myoho renge kyo, Namo Miao Fa Lien Hua Ching, Namas Saddharma pundarika sutra nor Devotion to the Scripture of the Lotus Blossom of the Wonderful Dharma is the name of the Supreme Law.
Even were Nichiren to have written the Daimoku in brail, it is written Namu Myoho renge kyo. Nichiren never wrote Nam Myoho renge kyo, not once in thirty years, not on his Gohonzons nor in his writings.
“And when the two characters for Namu are prefixed to Myoho-renge-kyo, or the Lotus Sutra of the Wonderful Law, we have the formula Namu-myoho-renge-kyo.” -- The Opening of the Eyes
"The jewels of this jeweled vehicle are the seven precious substances which adorn the great cart. The seven jewels are precisely the seven orifices in one's head, and these seven orifices are precisely the [seven characters] na-mu-myo-ho-ren-ge-kyo, the essential Dharma for the Final Dharma Age." -- Recorded Lectures of Niko [Ongi Kikigakai]
Nichiren on discarding even a single character of the Lotus Sutra:
"A single character of the Lotus Sutra is like the great earth, which gives rise to all things. A single character is like the great ocean, which contains the water from all rivers. A single character is like the sun and moon, which illuminate all four continents.
“Now the Latter Day of the Law, is the time when the seven characters of Namu Myoho renge kyo-the heart of the twenty-eight chapters of the Lotus Sutra that Shakyamuni Buddha who had achieved enlightenment in the remote past, along with the bodhisattvas Jogyo, Muhengyo and the others is to propogate-will alone spread throughout the country.”
"In the first five hundred years of the Former Day of the Law following the Buddha’s passing, only Hinayana teachings spread, while in the next five hundred years, provisional Mahayana teachings spread. The thousand years of the Middle Day of the Law saw the rise of the theoretical teaching. In the beginning of the Latter Day of the Law, only the essential teaching spreads, but even so, the theoretical teaching should not be discarded. Nowhere in the entire Lotus Sutra do we find a passage suggesting that we should discard the first fourteen chapters, which comprise the theoretical teaching. When we distinguish between the theoretical and the essential teachings on the basis of the threefold classification of the entire body of the Buddha’s teachings, the pre-Lotus Sutra teachings were to be spread in the Former Day, and the theoretical teaching, in the Middle Day, but the Latter Day is the time to propagate the essential teaching. In the present period the essential teaching is primary, while the theoretical teaching is subordinate. But those who therefore discard the latter, saying it is not the way to enlightenment, and believe only in the former, have not yet understood the doctrine of Nichiren’s true intention. Theirs is a completely distorted view."
"Thus a single word of this Lotus Sutra is as precious as a wish-granting jewel, and a single phrase is the seed of all Buddhas."
"Again, a single character equals innumerable others, for the validity of the sutra was attested to by the Buddhas of the ten directions. The treasures bestowed by a single wish-granting jewel equal those bestowed by two such jewels or by innumerable jewels. Likewise, each character in the Lotus Sutra is like a single wish-granting jewel, and the innumerable characters of the sutra are like innumerable jewels. The character myō was uttered by two tongues: the tongues of Shakyamuni and Many Treasures. The tongues of these two Buddhas are like an eight-petaled lotus flower, one petal overlapping another, on which rests a jewel, the character of myō.
The jewel of the character myō contains all the benefits that the Thus Come One Shakyamuni received by practicing the six pāramitās in his past existences: the benefits he obtained through the practice of almsgiving by offering his body to a starving tigress and by giving his life in exchange for that of a dove;9 the benefits he obtained when he was King Shrutasoma who kept his word, though it meant his death, in order to observe the precepts;10 the benefits he obtained as an ascetic called Forbearance by enduring the tortures inflicted upon him by King Kāli;11 the benefits he obtained as Prince Earnest Donor and as the ascetic Shōjari, and all his other benefits. We, the people of this evil latter age, have not formed even a single good cause, but [by bestowing upon us the jewel of myō] Shakyamuni has granted us the same benefit as if we ourselves had fulfilled all the practices of the six pāramitās. This precisely accords with his statement “Now this threefold world is all my domain, and the living beings in it are all my children.” Bound as we common mortals are by earthly desires, we can instantly attain the same virtues as Shakyamuni Buddha, for we receive all the benefits that he accumulated. The sutra reads, “Hoping to make all persons equal to me, without any distinction between us.” This means that those who believe in and practice the Lotus Sutra are equal to Shakyamuni Buddha."
"Therefore, a single word from one of these kings can destroy the kingdom or insure order within it. The edicts handed down by rulers represent a type of pure and far-reaching voice. Ten thousand words spoken by ten thousand ordinary subjects cannot equal one word spoken by a king."
"Similarly, though we gather together all the various sutras, such as the Flower Garland Sutra, the Āgama sutras, the Correct and Equal sutras, the Wisdom, Nirvana, Mahāvairochana, and Meditation sutras, they could never equal even a single character of the Lotus Sutra."
"Shakyamuni Buddha and the written words of the Lotus Sutra are two different things, but their heart is one. Therefore, when you cast your eyes upon the words of the Lotus Sutra, you should consider that you are beholding the living body of the Thus Come One Shakyamuni."
"The characters of this sutra are all without exception living Buddhas of perfect enlightenment. But because we have the eyes of ordinary people, we see them as characters. For instance, hungry spirits perceive the Ganges River as fire, human beings perceive it as water, and heavenly beings perceive it as amrita. Though the water is the same, it appears differently according to one’s karmic reward from the past."
The blind cannot see the characters of this sutra. To the eyes of ordinary people, they look like characters. Persons of the two vehicles perceive them as the void. Bodhisattvas look on them as innumerable doctrines. Buddhas recognize each character as a golden Shakyamuni. This is what is meant by the passage that says, “[If one can uphold this sutra], one will be upholding the Buddha’s body.”1 Those who practice with distorted views, however, are destroying this most precious sutra. You should simply be careful that, without differing thoughts, you single-mindedly aspire to the pure land of Eagle Peak. A passage in the Six Pāramitās Sutra2 says to become the master of your mind rather than let your mind master you. I will explain in detail when I see you.
“Now this Lotus Sutra of the Wonderful Law I have been speaking of represents the true reason why all Buddhas make their advent in the world and teaches the direct way to the attainment of Buddhahood for all living beings. Shakyamuni Buddha entrusted it to his disciples, Many Treasures Buddha testified to its veracity, and the other Buddhas extended their tongues up to the Brahmā heaven, proclaiming, ‘All that you [Shakyamuni] have expounded is the truth!’ Every single character in this sutra represents the true intention of the Buddhas, and every brushstroke of it is a source of aid to those who repeat the cycle of birth and death. There is not a single word in it that is untrue."
"Even if one were to prepare a feast of a hundred flavors, if the single flavor of salt were missing, it would be no feast for a great king. Without salt, even the delicacies of land and sea are tasteless."
“The Lotus Sutra of the Wonderful Law, before which I bow my head, in its single case, with its eight scrolls, twenty-eight chapters, and 69,384 characters, is in each and every one of its characters the true Buddha who preaches the Law for the benefit of living beings.”
"Every single character in this sutra represents the true intention of the Buddhas, and every brushstroke of it is a source of aid to those who repeat the cycle of birth and death. There is not a single word in it that is untrue."
"If one discards one word or even one brushstroke of the sutra, the offense is graver than that of one who kills one’s parents ten million times over, or even of one who sheds the blood of all the Buddhas in the ten directions."
I can supply dozens more similar passages.
Were believers to chant together, some chanting Namu Myoho renge kyo and others chanting Nam Myoho renge kyo, how discordant it would be. Some chanting Namu Myoho renge kyo and others chanting Nam Myoho renge kyo is one of the principle causes of Nichiren faith disunity. One may spout all the theoretical reasons in the world why Nam Myoho renge kyo is equally correct but reality and actuality trumps them all.
Throwing out the character Mu is to throw out a golden Buddha from the heart of the Lotus Sutra. Perhaps the SGI should remove the character Mu from their Gohonzons as they have from Nichiren's writings?