You don't even need to do a "rename" then "cancel". Just press tab to highlight the "rename" button and it will work. I tried adding sping missions, eras, and other odd things and it couldn't load the game. So even if you can add something doesn't mean it will work. I haven't tried starting a new game with a fifth era though.
I don't think you can add eras and civs--the game will not handle it. I've tried adding a civ and the game crashed right after clicking "New Game" or "Quick Start." When I tried adding a future era after the modern era, the game didn't crash, but I could never progress into the future era, even after researching all the modern era techs. So although you can use the editor or the Civ3MultiTool to add things, the game won't necessarily work if you try to add certain things.