If you:
• apply formatting (style) restrictions;
• define a new Section and configure it with your default heading/content arrangement; and
• create a custom Quick Part for the new 'Section', including both its preceding Section break and the paragraph break immediately before that;
you can combine that with editing restrictions throughout the rest of the document, so that users can only edit portions you intent them to (eg if you don't include paragraph breaks in the allowable edits, they can delete a heading's content, but not the paragraph itself, which kind of defeats their content deletion efforts).
Any new Section they insert via Quick Parts will be entirely editable, but at least they can't mess with the basics.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]