Thanks Paul, that’s very helpful. If you're confirming that document structure should be defined via the proper use of Styles, then what I need to achieve additionally is:

1. Password Protect the document enabling track changes
2. Only allow 3rd party to modify normal/body text in the document, but not change headings (ie. certain styles) as this will modify the document structure.

Is that possible? I tried removing the heading styles from the list under 'Limit formatting to a selection of styles', however this completely removed the style from the underlying document which is not what I wanted to achieve.