I think you bring up two different points -
Daimoku as Mnemonic device.
Daimoku as cosmic chant transcending all limitations, for lack of a better way to put it.
Some comments as to mnemonic -
Nichiren Shonin's teaching on the Daimoku is founded on Zhiyi's Fa Hua Hsuan-i (Profound Meaning of the Lotus Sutra). As you may know, in that work, Zhiyi explained that the entire meaning of the Lotus Sutra can found in the Chinese title composed of the five characters 妙法蓮華経. Without going into the structure of Zhiyi's exposition, he devotes much of the work to analyzing each character and relating it to the message of the Lotus Sutra. In this sense, I agree, the Daimoku as taught by Nichiren is a sort of mnemonic device. However, that does not dictate that the characters ought to be read in Japanese rather than Chinese.
妙法蓮華経 is a direct translation of Sad-dharma-pundarika-sutra. Each of the characters relates to a part of the Sanskrit title - why not use the Sanskrit title as the mnemonic? In fact, Zhiyi relates the Chinese title to the Sanskrit, and Nichiren cites to this in his own writings - particularly in connection with 妙 which is the Chinese translation of the Sanskrit "sad" (सद् ?). Again, we could as easily chant "NamoSaddharmaPundarikaSutra" - the meaning would be the same, and Zhiyi's teachings on the Daimoku would still make sense, more or less.
By extension, "Sublime Law of the White Lotus Blossom Teaching" would contain all the elements that could directly relate to Zhiyi's teachings without much reworking.
I don't believe that Chinese is some cosmic written language dictating that the title of the Lotus Sutra must be rendered in Chinese in order to convey the true and proper meaning.
I could be convinced differently, but as it stands, the mnemonic nature of the Daimoku is not compelling to render the Daimoku in Japanese an absolute.
Morever, I find some support in Nichiren's own writing for calling the Daimoku in Japanese a convention. In one text, I can't remember off hand, Nichiren declares that his 5 or 7 character Daimoku is the same as Bodhisattva Never Despise's 24 character Daimoku which roughly translated is, "I would never dare despise you because you are bound for Buddhahood!" The words, Nichiren explained, are different, but the meaning is the same. By the same token, the point Nichiren takes from Zhiyi's Fa-Hua Hsuan-i is that the meaning of the 80,000 some odd characters of the Lotus Sutra are contained in the 5 characters of the Daimoku. The difference is that the particular causes and conditions of this Saha world, at this time yield the Daimoku in Japanese.
In saying that the Japanese Daimoku is a convention, however, does not mean that it lacks reality. We are operating within the context of East Asian Lotus Buddhism which takes as a given the principle of the Three Inclusive Truths, which tell us that all dharmas are real, even as they are empty, and even as all dharmas are empty, they are real. I hope I'm not going too far off an confusing my argument. I don't disagree with your proposition. The process you describe triggered by the rendering of the Daimoku in Japanese could and probably does play out the way you describe. However, that does not preclude the meaning of the Lotus Sutra manifesting and enlightening beings in other ways - this I believe is the teaching on Upaya which is one of the core teachings of the Lotus Sutra, going hand in hand with the Life Span of the Buddha.
Calling the Daimoku in Japanese a convention does not demean it. It is merely a description of its nature in accord with Buddhist, particularly, East Asian Lotus Buddhist thought.
As far as Daimoku in Japanese being a cosmic, absolute truth, I think the above comments about its conventionality address this. What if I was a being that communicated in scent - like many organisms? Would that in itself preclude me from understanding the nature of reality?
I hope you don't think I'm too direct or harsh - but if you're going to propose some limitation like the idea that the Daimoku works optimally only in Japanese, there has to be a good, articulable reason for placing that sort of limitation. As of now, I just don't see it.