1. You have to "Run" the script to get it Registered. After you run it (or set it up as Autorun), it will show up in the Registered View of the Script Library palette.
2. This is a built-in FrameMaker ExtendScript function.
3. Different notifications may use different parameters. For example, FA_Note_PostSaveDoc will set sparam to the file name of the file that was just saved. The object is useful if you need to pass it to a function. For your script, object will be the Doc object of the target document. If you don't need a particular parameter in your script, just ignore it.
4. You should post your code so we can take a look. I tested mine and I get the command number.
5. I don't know; this may be a bug.