As more and more missions are made, the worse this problem will become. I hope they try to implement Superfluid's suggestion.
The readme.txt file in the Missions directory actually says:
"... You can also place downloaded custom scenarios in this folder and play them from the Scenarios list."
Now we just need a button to give us the option to download custom scenarios.

O, I see scenarios that I loaded once, will now work when I go offline.
Thank you George_. Since I take my PC home over the weekends were I do not have internet. My weekends promises to be a lot more fun.