Hi April, welcome to the org

a) I think the single player modes are: Campaign, Custom and Historical. Campaign, if i understand, is where you pick a clan and go through its story, yes? Historical is like a scenario where you go through a famous battle. What's custom then?

b) In the Campaign mode, they say that your clan leader can level up. Is it a smooth transition where you go from beginning to end, like it's one whole entity, or is it mission based, like Starcraft, where you go from one "map" to another? Or is closer to something like Dynasty Warriors, where each mission can be replayed and you can spend the time to level up your clan leader?

c) I've heard that modding is disabled, but i also see modding in this forum! Is it difficult to make your own mods?
Custom battle is where you can pick an army and play against the CPU in a battle on a map that you choose. Its kinda like the fights in the campaign, but limited only to that map.

The campaign is.. well, its much more open ended from things like Starcraft or Dynasty warriors. You should play the Shogun 2 demo which is available on steam. If you've never played a total war game before it can be a bit different from what you may be expecting. Think of it like civilization, only a lot more straight forward, with the ability to fight the battles yourself rather than rely on number crunching.

Modding is a pain from most reports, requiring hex edits to do much of anything in the game. Installing mods, however, is pretty easy. And there's already a number of good mods like Shogun 2 realism, Darthmod, and Road to Kyoto