Yes, I think we are on the right track. I did notice he wrote Naomi
in phonetic characters, as though it were a transliteration--the same way Risa and Erika
(and other originally Western names) are still written. But when Naomi
became a popular name in Japan a couple of decades later, the spelling that hit the top 10 was ’¼”ü. So clearly it had been reinterpreted by that time as a meaningful Japanese name.
The character “Þ has been used phonetically in place names (for example, Canada) for a very long time. In first names, I'm not sure. In the 19th century it was more common to write girls' names in katakana or hiragana, especially if they carried no suffix. Which reminds me, Kimi, Tomi
and Fumi were all still popular up to the 1920s, so even though they aren't examples of "mi" as a suffix, girls' names ending with this syllable were definitely around. It may very well be true that Naomi
was first used in Japan in this novel and that the idea of using "mi" in place of "ko" was a new one, but it was a relatively small leap. And an extraordinarily successful one. =)
ETA: Argh, it wouldn't let me cut and paste the kanji from the last post, and I don't have time to redo it. Sorry about that!