1. The fact that Soul Society is clearly Japanese is due to Kubo's nationality and stands as a kind of logical mistake? I mean, it's not only that everyone dresses Japanese way, talk Japanese, but there wasn't even one character that had none-Japanese name. Or are there more Soul Societies? Like, for Europe, America etc.? Or is it universal? If so, then where are people of other countries?
2. I know shinigami live a long life, but are they actually immortal? If so, then they cannot have children - mortality drives reproductive capacity, that's how it is. But seeing the mechanism of noble houses... Byakuya had a grandfather, so clearly we see generations and thus the ability to bear children. So maybe the ammount of births is controlled? Like in China? And if it isn't, but shinigami are still immortal, to balance the births and deaths, shinigami would have to die by the same number as children are born. And it may be it, seeing as they die a lot and because they are soldiers, they don't really think of having kids - but the hectic time Soul Society has right now is due to Aizen, which only lasts for two hundred years. What about earlier, then?
That's not a problem if they are indded mortal - they age, after all, and somehow, it doesn't sound fun to remain an old man/woman for eternity.