Thanks, macropod, for you helpful reply. Turns out that creating a separate Notes section solved by:

1. Create new empty section after last chapter
2. Suppress endnotes in all chapters, but not in the new section

Suppressing headers in sections not a well advertised technique from my Internet searches and Word Help searches. But I found this very helpful guide by the University of Wisconsin.
https://lafollette.wisc.edu/images/p...placement.docx
To suppress endnotes:
1. Click in a section.
2. Go to Page Layout: Margins.
3. Pull down to Custom margins.
4. Click on Layout tab.
5. Check suppress endnotes.
6. Repeat for every section except for the last one that starts with the continuous section break after the endnotes header.

Solved the back matter headers problem by: unlinking each section's first odd and even pages from previous section, keeping the Different Odd & Even pages setting, setting each section's odd and even pages to the same header.