I'm not sure I understand the question. The 'Heading 2' styles are inserted using Word as a wordprocesser. Once you've saved the rtf/doc/docx as html, if you want to edit the HTML in a text editor, personally I'd use my favourite text editor and that wouldn't be Word. This is not to say that Word can't be used, it's just that I have more faith in Notepad++ as a text editor. However, since I'm a Windows user not Mac I don't have a Mac suggestion.
Word can produce truly horrible html if you let it. Saving as 'webpage-filtered' reduces some of the MS garbage. I seem to recall someone saying that 'webpage-filtered' is called something else in the Mac version of Word.
Another way to keep the html cleaner is to create/use named Word styles and apply them to your paragraphs when you're trying to change font-size, margins, indents
e.g. if you apply style 'Normal Web' to a paragraph, the html output will be a nice clean
<p>
your paragraph text</p>
If you highlight paragraphs and then start changing margins, indents, size etc you get something more like
<p style="font-family:"Times New Roman"; font-size:12pt; text-indent:18pt; margin-top:6pt; margin-bottom:6pt">
your paragraph text</p>
and that styling code will be repeated on every paragraph you changed.
I believe Calibre is good enough to sort out the MS mess during conversion but if you're going to use the HTML file to clean up any typos/errors you find later, a clean HTML file is much easier to see what you're doing.