I'm sorry to hear that. Its good practice to click the save button each time you add a new element to your mission, and if you do that internet problems wont really matter. However, they will not likely make any offline part of this game since that is too open to hacking.
I'd like to make the suggestion that the toolset be made available offline. As I know jack squat about programming and what not, I don't know what the ramifications of this would be, other than Nagus' indication that this would open it to hacking. Still, if it's reasonably possible I'd like to see it happen.
As an example, I was on a flight a couple of days ago and thought to myself that it would be fun to be working on a UGC mission. I ended up jotting ideas and outlining plot, but how much better could it have been if I'd been able to actually do some of the "programming" work?