So the question is — what was really intended by these warnings not to engage in slander of the Lotus Sūtra?

As Nichiren has explained, the Lotus Sūtra’s two unique teachings concern the One Vehicle whereby even those who would seem to be excluded from attaining buddhahood are promised its attainment and the revelation that Śākyamuni Buddha had in fact been the Buddha since the remote past even before his awakening beneath the Bodhi Tree. Women, evildoers like Devadatta, and those disciples who were believed to have become arhats who would no longer return to the world after their passing, are all told that they will in fact return to the world and attain buddhahood. This was in seeming contradiction to the earlier teaching that only a very few could aspire to and attain buddhahood. The revelation of the attainment of buddhahood in the remote past means that even during the Buddha’s innumerable past lifetimes as an ordinary human being, or an animal, or some other form of sentient being striving to attain buddhahood the Buddha had been a buddha all along. And now even though the Buddha is going to appear to pass away for good, he asserts that he will still be present. In light of these two teachings, buddhahood should be understood as inclusive of all beings, all time, and all space. It is a constant and active presence even when it is not apparent or seems to be absent in the lives of those who strive for it. Throughout the Lotus Sūtra these ideas are put forward as the fullest expression of the Dharma and to embrace them with faith and joy is to embrace the Wonderful Dharma and to reject them is to reject the Wonderful Dharma. The Wonderful Dharma is held to be even more worthy of respect and offerings than the Buddha himself because it is through the Wonderful Dharma that one attains buddhahood. It is for this reason that rejection means a total alienation from what is truly of value in life, and therefore leads to rebirth in hell. It is for this reason that a single moment of faith and rejoicing in the Wonderful Dharma of the Lotus Sūtra is said to bring unequalled merit, rivaled only by the merit brought by the perfection of wisdom itself which is none other than buddhahood itself.