As I think on this further, even these voiceless /s/ and /sh/ sounds involve putting the lips into either an /u/ or an /i/ shape based on the following vowel ev

As I think on this further, even these voiceless /s/ and /sh/ sounds involve putting the lips into either an /u/ or an /i/ shape based on the following vowel even if that is also voiceless, creating that which is not a syllable in English, but perhaps is for this purpose in Japanese. The C-V cadence and final vowel (given lack of final -n) rules are satisfied...