Linking Table of Contents

If there were 120 chapters, the use of a script would be justified.
As there are only 12, you'll go faster by manually creating a link to each of the 12 chapters on TOC.

See: https://helpx.adobe.com/acrobat/using/links-attachments-pdfs.html

For the link back to the TOC you'll need to do the same, or use a button-type form field that you'll just need to copy and paste onto each of the 12 chapters.
(Unlike a ‘Link’ object, the ‘Button’ form field does not work in all PDF readers, but unlike a ‘Link’ object the ‘Button’ form field can be copied and pasted).

The other, simpler solution would be to use Bookmarks and set the document to display the Bookmarks panel when it is opened.

Search for "Create a bookmark" on this page: https://helpx.adobe.com/acrobat/using/page-thumbnails-bookmarks-pdfs.html

The first part is much trickier. You would need to use a script to read the contents of those pages, identify the page numbers (probably based on their location on the page, if they are aligned to the right), and then create buttons on top of them that link to the destination page. This will require the development of a custom-made script.

The second part is easier. You just need to create the "Go Back to TOC" button on one page, and then duplicate it to the rest. The duplication can be done manually, or again, using a script.  You can also use the built-in Create Duplicate Fields command, but that duplicates it to a range of pages, so then you'll need to delete those that don't apply.

If you're interested in hiring a professional to create all of this for you, feel free to contact me privately by clicking my user-name and then on the "Message" button.

Nearly forgot to mention that it can NOT be converted to Word ... it's part of a music ed book and it's all notation files - the cover and intro section with TOC (five pages) and first page of each chapter are docx files, but everything else is PDF.