Before I make things worse (registered second base game code - game won't start)

I hadn't tried that yet, thinking something in there was causing a conflict. So, I just did, and the short answer, I guess, is it doesn't work. However, it's not quite that simple. The first time I tried to start it, it loaded right up but without some of the content for the save. Thinking things had unchecked themselves in the launcher, I quit and restarted the launcher. Everything was checked, so I started again, and the game won't start.