Those are simply the scenarios that will be in the game when it is released -- we will be adding more scenarios over time, and not charging a thing for them. My plan is to have at least one scenario ready to go for our first patch, and quite possibly more (mainly depending on my non-WCS workload).
You're certainly right about the "amateurs," but I think part of the fun -- or perhaps "challenge" is the better word -- is trying to win with inferior forces bearing inferior weapons. By the time one gets to Gettysburg the troops are as good as they were ever going to be and their weapons (with a few exceptions that never had wide distribution) were also as effective as they were going to be. BAB is designed with both types of armies in mind. It should definitely be interesting to compare the experience of fighting 2nd Manassas and 1st Manassas.
I have to say, I didn't know much about this early phase -- other than the basics about 1st Bull Run -- until I began to research the battles, but now I find it a fascinating period. The Battle of Williamsburg is one of my favorite battles of the Civil War, and it surprises me that it's been so overlooked, overshadowed as it's been by the Seven Days' Battles two months later. Wilson's Creek is also very interesting, as is the background. And Mill Springs is a tiny battle by Shiloh or Gettysburg standards, but still quite compelling. (If you get the game you will find in the manual that "Battleline," one of the two main contributors of generals' bios for "Forge of Freedom," has written lengthy summaries of each battle that do a good job explaining their circumstances and impacts as well as how the battles unfolded -- definitely worth taking a look at.