Headings inside table cells don't appear in Navigation pane: workaround?

This is a known issue and Stefan Blom's answer still holds:

It is a known limitation in Word that headings inside table cells won't appear in the Navigation pane. For what it's worth, the same thing applies to the predecessor of the Navigation pane, the so-called Document Map, in older versions of Word. As you have noticed, Word properly adds headings inside a table to the TOC, fortunately.

If you want the headings to appear in the navigation pane you have to place them outside of tables, i.e. your suggested approach is the way to go.

If you still want the heading to appear like it is part of the table you can simulate that by adding a paragraph border around your heading. (Note: This still does not work if it is in a table cell. You must be outside the table cell.)