Could it be that the English/Latin-based letters "AA" "II" and "UU" that are included in the list are just transliterations of the Napanii characters?
I do think that absolutely makes sense. In fact, while I thought about it I totally came around to thinking that's better. However, following that logic, I think the other vowel pairings (including things like IE, OU, etc) should also be single letters in Napanii.
Actually, looking at it I now realize you have only made the double AA, UU and II into single letters. The same should follow for EE, OO and YY. It might be that those don't feature on system names, but I don't see why they wouldn't in the general language.
One more bit of critique/thinking, too. I think it might be better to have the vowel letters have some internal logic to the way they are marked. Or it totally could be that I'm just not seeing why you chose those particular variations on the basic I-shape to depict each vowel. I think the additional symbol added to the basic I-shape should somehow reflect the nature of the sound in question. For example, your letter for A feels like it should be O instead because it is a round, all-encompassing sound. A single vowel letter might use a shorter, more curt designator while a long double vowel sound would use longer, more flowing symbols. That kind of linguistic logic seems to appeal to me, in any case.
Edit - Added:
After further thought on this matter, I thought of a couple of more questions regarding the fictional Caldari language, Napanii. Of course, any human-based language in New Eden would have descended from a language/languages spoken on Earth. However, that does not mean (in my humble opinion) that they would be identical to any language that was spoken on Earth 16,000 years ago. It may share striking similarities to certain languages, especially the ones that most greatly influenced it...but it would not be those languages, just like Spanish, Portuguese, Italian, French or Romanian are not Latin, just like English is not German...they may descend and have been influenced by certain tongues, but they are something new, and thus to explain how the language functions say in to an English-speaker (for example), it may be required to show certain things in a way that the English-speaker will understand. Thus the concept of double-vowelled letters.
I agree totally.
It's just pretty amazing how exactly like Finnish the Caldari in the game is (I don't mean "kind of exactly", but rather "totally exactly") and since it is also a cited source for the Caldari, I'm offering my advice here with a fair bit of emphasis on that similarity. While most players struggle to pronounce Caldari system names, Finns generally have no problems at all communicating those (on Vent, or whatever). Taking things further than those origins and in different directions is cool though and indeed should be emphasized, I feel (indeed, I'm usually the one on the boards preaching about why we shouldn't just look at Eve cultures as simple extensions of singular earth cultures, like Intaki->India or so on).
btw, "Saisieni" is "Gotmushroom" in Finnish. Ahem.