Yes, the game can be played offline with bots. No, you don't have to install them, just choose single player and the map you want to play and you're good to go.
I'm not sure about the latest patch but they used to limit the maps you could play on to the 16 player maps, but you can mod the game to play with more bots on the larger maps if you want to and your computer can handle it. There are also user made maps you can add if you want.
I actually enjoyed playing single player more than multiplayer, the constant jumping and diving and other odd movements you'd never actually see someone do in a real battle spoil the battlefield simulation aspect of the game.
There are some problems with single player. For example if you camp out anywhere the other side will call in an artilliry strike on your position. But overall single player is still pretty fun.