1. Possible to fly without a joystick, but quite difficult, especially with the realism turned up. With the realism turned down, it should be more doable.

2. There are US campaigns, but they are Pacific based against the Japanese - not Europe based against the Germans. There are some single-missions flying American versus German however. There are Russian, German, U.S., British, Japanese, Finnish, ... lots of campaigns available and lots of single-missions for every country too. With all the expansion packs that have been made (all of which which the GOG version has automatically) and content patches the developer has released (GOG doesn't have the most recent one, but they are free to download) you can fly a huge number of planes under the banner of many different countries that took part in WWII, both large and small.

3. Some campaigns are set, some are randomly generated campaigns (each time you play the computer will randomly generate a set of missions so you never play the same thing twice) - I've not played any of the campaigns, so I'm not sure how much of a story they actually have. I think the new "Il-2: Cliffs of Dover" - which is a new game rather than an expansion to the series on GOG - has a character-based storyline campaign, but the last I heard is that the game is very buggy. Hopefully they've ameliorated that.

IL-2: 1946 is a lot of fun, but I really would strongly recommend a joystick.