1. Have the strings in add.str.txt been added to a file, are they (therefore) obsolete or does the game use those strings?
2. If I have to add a new string (because I added a new government, for example), does it matter where I put, or could I just put a well-named string in any file or maybe even a new file such as newgov_str?
You can put new strings in any file you want, but if you make a new file with the strings for your mod you should also add the name of that file to strings.txt.