ePub issues Fixed and Reflowable no showing as it should

Export to EPUB is not a simple process, like printing to paper or exporting to PDF. The source document has to be optimized for e-book export, which means, put briefly, that it has to have meticulous structure, organization, use of styles, etc. Many things that will work in print, or at least won't cause problems, will have poor to disastrous effects on an EPUB export.

Fixed-page EPUB/FXL is a problematic format and should be avoided. If you're going to polish your book for EPUB and learn the steps, it's far more worthwhile to do it to a reflowable format end, which is the proper format for all but a select set of e-books.

EPUB readers vary enormously. Hardly any two will render a page, or a book, the same way. Sometimes the differences are minor, but a single book can range from "flawless" on one reader and an unreadable mess on another. It's important to use a plain, standards-based reader to do your proofing if you are releasing the book to a general EPUB readership. Calibre is currently the best; if Thorium Reader solves a critical font-sizing bug, it will return to being the gold standard for EPUB readers.

Generally, text flow is no problem for export, no matter how many heading levels you have. They do need to be styled appropriately, especially with respect to font size, weight and spacing, just as they would be in a print book. Images are not hard but they MUST be anchored in the text, usually to the end of the paragraph they follow. Don't attempt any fancy formatting; just center them on the page with no text flow on either side.

The really big rule is that everthing must be in one text flow. If you have text broken up among multiple unlinked text boxes, or any element just 'stuck on a page' and not anchored to adjacent text, it will either not export or will pile up at the end of the document. One flow throughout, with all elements like images and text boxes anchored. This includes a TOC text box if you include one in the text; that's a bit out of date and a deeper dynamic TOC managed by the reader is a better idea.

That should get you on the right path. Happy to answer further questions.