[andrewas]

Chitin.key is an index into the .bif files. Some mods alter this file to index into additional .bif files, which has the potential to cause problems. Restoring the orignal .key cuts the indexes to the new .bifs, so they have no effect on the game.

Bgmain.exe is the actual program. Breaking this is fatal. Very occasionaly a mod will alter this, in order to change on of the hardcoded values in the game. The starting exprience patcher is the only one that come to mind. (The no-cd patch also changes this, but that isnt a mod).

Baldur.ini is configuration information. What resolution to use, which movies are available, what size install you are using, that sort of thing.