Japanese definitely has vowel length. The pronunciation as two short vowels is only really heard when singing*, otherwise, they're pretty much pronounced like long vowels, and there's even the vowel lengthening mark (which looks like a dash). Furthermore, for the most part, even the clusters ei and ou have reduced to long vowels, ee and oo, respectively, though some dialects may retain the cluster.
* In singing, I notice that oo, regardless of where that occurs, is usually sung as owo, which is likely a result of the o-wo merger, which is also why the wo particle is now pronounced o. I believe wi also merged into i, while we merged first into ye, then ye in turn merged into e. I need to see if these leave any traces when singing. Also, some Japonic languages go further and even merge wa into a, which is why, for example, Okinawa is Uchinaa in Okinawan.
Now, I do agree that long vowels are two morae long, but that does not negate them being long vowels, and the same is true about geminate consonants as well. And there just happens to be an Indo-European language that shares this property - Latin, where long vowels and geminate consonants were in fact origially wrote as double vowels and double consonants, then the apex was introduced for both, but it only stuck with long vowels, while it fell into disuse with geminate consonant which ended up being written as double consonants again. Japanese, on the other hand, writes long vowels in a variery of ways, including with the vowel length mark following the mora and writes geminate consonants with the geminantion mark (the small tsu in both higarana and katakana) before the mora.
Furthermore, Japanese vowel length is acknowledge by the Hepburn transliteration system, which uses the macron for the purpose, which just so happens to also be used in modern Latin transcription to replace the apex.
Now, the moraic n is indeed a peculiarity of Japanese, though in speech, it is basically pronounced like a normal n (or m or ŋ if preceding p/b or k/g, respectively), the peculiarity only arises in singing. That, lenghtening the correct vowel, and the correct shortening/near-elision of short u and i, are the three pitfalls I've observed non-Japanese speaker stumble on when singing in Japanese.
Incidentally, that short u and i that get shortened to the point of near-elision, seem to me to parallel the development in Slavic where the two yers (which were also ŭ and ĭ) developed the same way and in fact, in most Slavic languages, got completely elided in the end, which I think would have happened in Japanese as well, had it not been for the writing system not allowing such a loss, though it does seem that way in speech at least, eg. desu is often pronounced essentially as des, and yoshi as yosh.