Lets use Acupuncture by Meigs as an example. The student version and the teacher version have two separate numbers because the content is not the same. The annotations, etc. make it a separate volume. For most titles, the student number is on the copyright page of your teacher's copy. No, except as a point of reference.
Packages have different ISBNs than the book itself.
Packages differ in content and price, so the number must be unique. Books in Print will help you identify the correct item. Yes, with one exception. For most of titles, the new edition will always be assigned a new number. When the movie version becomes popular, Signet realizes that Hugh Grant's face slapped on the cover may just sell more copies.