When you say you can't get FOC to start, what do you mean exactly (i.e. is it crashing, not opening when you try to run it, or are you just getting EAW every time?) and how are you trying to launch it within Steam (desktop shortcut, taskbar or from your library)?
All of those are good questions that in all of my edits I really should have detailed. So for starters, I'm attempting to launch it through the sidebar on the left (as I don't think you can launch FOC from hitting the play button in the center). Now I've double clicked it and it's brought up the options for Vanilla or FOC, selected FOC, and that didn't work. I've also tried right clicking on the game and selecting it but to no avail. Now as to what it does; when I select FOC to play, it'll say next to the game in the sidebar "running" for about five seconds and then that goes away and the game never launches.
And to be super clear, though I think I said this already, I can launch the game by going into game directory and launch the game just fine there. The problem arises that when I attempt to relaunch the game to activate the mod, the game does exactly what I described above like I had just tried to launch it for the first time.