Those are great questions. Unfortunately, I'm on my mobile phone right now, so I can't give you more detailed answers at this point.
Regarding the architectural styles of Shirogane homes, the first thing to note is that they're mostly inspired by those found in Edo Japan (1603-1868). The mokuzo (木造) style is the ubiquitous wooden home you'd find across most of the country even today. The kurazukuri (藏造) style is inspired by merchant homes and warehouses typical of the mid- to late-Edo period. Great examples of this type of building can still be found in Japanese cities like Matsumoto, for example.
The nanpu (南風 or "southern wind") style takes its inspiration from the architecture of southern Japan, particularly Okinawan architecture.
EDIT: I just learnt that 南風 / "nanpu" has an older meaning of summer. So, nanpu style could literally mean "summer style".
If you've been following the news today, you'll have heard of a massive fire in Naha, Okinawa, which completely destroyed the main keep of Shuri Castle. Look up the pictures of the building on Google, and you can immediately see its similarities with the nanpu style in Shirogane. The Ryukyu Islands (what the kingdom of Okinawa used to be known as, before it was annexed by Japan), were commonly known as the southern islands by mainland Japanese, hence the reference to "southern winds".
As for Hinganshi itself, a look at the map will reveal two main islands, and yes, Kugane and Shirogane are in the south. Kugane, in particular, is the FFXIV analogue of Edo-era Nagasaki.
Being a trading port, Kugane is likely influenced heavily by Doman culture (just like how Kyushu and Okinawa were heavily influenced by China) hence the eclectic mix of "Chinese" and "Japanese" styles in the city. So, in a sense, I don't think Kugane is very representative of the rest of Hinganshi itself. I wouldn't use it as a reference on what might pass for a typical "southern" style of Hingan architecture.