The easiest way is to let Calibre auto-detect the chapters. See Structure Detection
and Table of Contents
. By default, calibre will assume chapters start at either <h1> or <h2> tags that have any of the words (chapter, book, section or part) in them or that have the class=”chapter” attribute. I assume MS Word Headings get mapped to heading tags as part of the DOC to HTML export. Since you have control over the document, changing the chapter detection to all <h1> or <h2> tags (e.g. if your chapters don't all start with Chapter) or using a multi-level setup should be relatively easy to make work.
Usually, .mobi and .prc are interchangeable filename extensions (i.e. just rename the file from .mobi to .prc). The only exception might be if you are targeting very old handheld devices, which might have some image size restrictions.