I am really looking for the forensic limitations of Exif & Metadata;

1. Can the Exif/Metadat data be changed manually in photoshop 3?
2. Is it changed automatically when certain edit functions are used? I speak here of the picture being taken day especially.
3. Is it possible to add captions and other data to the picture without the picture taken day being changed?
4. What are the forensic limitations of Exif/Metadata?

Thanks-
Phil