There are several stages to writing, that need to be separated out.
NO application can do all three well enough for a 100-page doc.
I use a web-app called WorkFlowy for all my outlining. It is quite simply brilliant. Possibly the best app I have ever used, for simplicity, features, and utility. Also works brilliantly on mobile.
I write in FullScreen No Distractions mode in iaWriter. This is a no-frills writing environment that gives you a white screen, black text, and zero formatting tools. For serious writing, I can't imagine using anything else, any more. It keeps you focused on the WORDS, and not the FORMATTING, and ensures that whatever you write holds up because it is good words, written well, and not because you used fancy italics and listicles to hide the fact you don't know what you are talking about.
Finally, once I have all the content written, I put it into ... gasp! ... Microsoft Word.
The only way to win any battle with MS Word is to fight on it's own territory; use a simple, built-in template, and use the built-in styles.
So select a template. Flow your text in from iaWriter. Then start formatting it, start to finish, using the clickable default formatting options.
When inserting images, use a PNG and insert it via the "Insert" menu item. Use the Image Formatting pane to set the location, style, and attributes of the image.
I output the Word file to PDF, and print the PDF, because this solves some of the more annoying Word printing issues. But the key with Word is to not try and do anything with it that the programmers haven't already thought of and created a button for. Fancy formatting, inserting a sidebar, and using your own custom stylesheet will ensure failure.