Yes I'd say the difficulty level is comparable to WoE, perhaps a little more complex.
If you're wondering we have the Georgia/Black sea Caucasus region at the moment which is pretty big. You have the ability to make your own missions down to a minute level of detail (compared to WoE where the "editor" sucked").
1. Yes, on different computers.
2. Kind of, you can fly a series of linear missions but they won't be saved as a campaign as such.
It's worth checking out the free DCS world with the Su-25T for a bit just to check on FPS and such before you spend any money.