Only a new HTML file works for mobi, unless you put in ancient mobi specific code. Thus new HTML file works for Amazon making an old mobi, kf7and kfx in the same epub file uploaded to Amazon KDP as everywhere else and no page break HTML code at all. No HTML edit needed from a docx source in Calibre and only image CSS needs edited (for % and auto or vice versa on height & width on larger images).
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