This is purely my personal design preference but I figured I would throw it out there anyway. In my campaigns, I tend to try and start with a smaller, easier battle for the player to get help get them into the story without them running into too many casualties with smaller "catch their breath" scenarios placed throughout the campaign. Its not an easy thing to do as it will always be harder for some than it is for others due to experience, gamey tactics, etc. Too hard and people get frustrated, too easy they get bored.
In this campaign storyline though I don't really see a way to start the player small and then ramp things up since they have to go straight into the landing. Something you may want to consider is 2 distinct campaign branches, one harder, one easier and let the player choose in a pre-game scenario the path they want to follow. Its more work but not as much as you might think since you are doubling up the use of maps (a big time consumer) and the friendly core force only needs to be included in the 1st scenario.
I use an Excel Spreadsheet and Access Database for managing campaign data when I build them if you have those programs and are interested. In the DB, you can generate all the campaign text files from a single click BUT they are not real intuitive and there are no "instructions" how to use them. You may have to manually go through each file afterwords and clean the editing of each file to make sure its looks good in the game interface though.
