Forgot to mention this important fact about my workflow:

I do NOT convert the Sigil ePub to a mobi via Kindle Previewer. You can certainly do that of course. Rather I unzip the ePub and basically modify the content.opf file a little bit. When I compile the book I point Kindle Previewer to the content.opf file, not to the ePub file. So basically Dreamweaver is the heavy lifter, the tool I use the create and edit the text and initially lay out the book structure, then finalize the book. Sigil is merely used to add a cover, edit the metadata, add a TOC, and create the content.opf and toc.ncx files which are quite frankly a PITA to create manually. Everything I use Sigil for I could do manually, but Sigil saves a lot of time and is way more accurate than my manually created files would be.

You can of course totally create your book in Sigil then have calibre or Kindle Previewer convert it to mobi. That would be much quicker and easier. For personal documents this makes a lot of sense. But for published books I desire more control.

EDIT:

• Think of the content.opf file is being the blueprints from which to build your book. It lists all the files, lists where they are located, defines what they do, and how they should be used to create the mobi book. It also contains metadata for the book.
• The toc.ncx file defines the TOC and structure of the book.