I can try and answer some of your questions. No changing weather, especially up on the mountains, always snowy. My sims go there for their summer vacation to ski.
There are Japanese sims everywhere.
Lifestyles are depended on the sim. If they like to spend a lot of time outdoors, they will end up with outdoorsy lifestyle. These lifestyles are not carved in stone...they can change. When a sim has the Single and Lovin It lifestyle, if they were to get into a relationship, they will have second thoughts. Other sims aren't usually affected by those lifestyles, just the sim it applies to. There are all different lifestyles. If you wanted a particular sim to have that lifestyle, just make sure you do things that would cause it.
The big lot on the mountain would be a great place to put down a palace. Unfortunately, you would also have to bulldoze the Onsen Bathhouse to get to another large lot. Not sure that is a lot you would want to get rid of. Someone else may have a better answer.
I hope I answered some of your questions. Happy Simming! <3