As I wrote about my grandfather, the first observation - my skepticism - has been the view I've held for the past 25 odd years. The second part - that maybe the limited practice he performed was all that was required to receive such profound benefit as the Sutra promises - dawned on me as I wrote it. Your point that we can't, as ordinary beings, know the spiritual achievements of others, further undermines my previously resolved skepticism. Thank you.
In a Nichiren-Lotus household, Amitabha oriented practice would be perceived to be a heresy resulting in severely detrimental karmic retribution. I don't know what my grandfather's practice entailed or if such a detail would have held any significance for him. He may have been contemplating Amitabha for all I know. His actual family background was Shingon, which does encompass some Pure Land practice, but given this, he could have also been practicing devotion to any number of other Buddhist tutelaries, or just some general worship of "Buddha". I don't think my grandmother would have approved if she knew that he was engaging in Pure Land practice - her family background was Pure Land which she deeply repudiated. Then again, she was broadminded enough that she seemed to just be happy that he made offerings to the Buddha, who cares about the details. I mentioned the incense offerings - my grandmother knew he made those offerings because she would find the unburned ends of the incense sticks in the ashes - other Japanese Buddhist traditions burn incense vertically - in Nichiren practice, we generally lay the incense horizontally which allows the sticks to burn completely. That's just to say, maybe he was doing his own idiosyncratic practice at the family alter.
In Nichiren Lotus Buddhism, practice generally starts with the approbation (adhimukti) of the Primordial Buddha - specifically, the Life Span, but more generally, the full body of the Buddha, the Buddha's complete interpenetration with ourselves, and complete devotion (sraddha) to this teaching which is embodied in the Lotus Sutra. This does not necessarily entail present understanding of the Primordial Buddha, but something more like an acknowledgement (adhimukti precedes and does not include understanding). Faith in the Buddha's grace (prasada?), while arguably featured in some respects, is different than the grace you have in Pure Land. In Pure Land, my understanding is that one is wholly reliant on the vow of Amitabha, which is redeemed through faith - whether a single moment or a sustained state of mind. In Nichiren Lotus Buddhism, emphasis is on awakening to Buddhahood here and now - possible only if you are aware of the Primordial Buddha - and sharing this teaching with others in real, practical ways, in this life. As Nichiren taught, we strive to read the sutra with our bodies, here and now, which entails carrying out the Buddha's work of spreading the Buddha's teachings and enduring the hardship that accompanies carrying out this work. We are grateful for the Buddha's compassion in teaching but in line with more mainstream, traditional Buddhism, we honor the Buddha by putting the teachings into practice and striving for Buddhahood now and helping others to do the same. Nichiren Buddhism is actually critical of the passive approach reliant on grace you often find in Pure Land practice which in some interpretations, especially those spreading at the time Nichiren was alive, writes off the efficacy of Buddhist practice, aside from Amida faith, here and now.