[thaumasiotes]

> The pronunciation has always been “tsu”.

This seems unlikely. The pronunciation was tsu before WWII. But the organization of the syllabary strongly suggests that the pronunciation was once tu.

I don't know to what degree the syllable 'tu' is viewed as impossible in Japanese as opposed to merely nonexistent. (Compare Mandarin, where (as in Japanese) there is no syllable /si/, but it's not especially difficult for Mandarin speakers to pronounce /si/.) I'd be interested if you knew.

[layer8]

From what I’ve read, it’s been “tsu” at least since the Heian period (so for roughly a millennium), and there is no clear evidence that it has ever been “tu”.