Splitting an existing work into components is a well-documented feature (I believe it’s mentioned in the first of the introductory videos linked to under Help in Scrivener). There’s even a feature to split on a particular string of characters (often “Chapter”), so that you can start with at least a partial breakdown.
Doing this with an existing manuscript will give you some insights into how Scrivener’s compile function works, what it does for you automatically (like adding “CHAPTER ONE” automatically, instead of you having to type it into your text and number it sequentially). It also will show you how the organization of your manuscript affects that output, and some of the things that may not seem obvious to you, like how to keep your title page and acknowledgments pages from getting chapter headings.
So I’d say go ahead, if for no other reason than the learning curve. Once you’ve figured this stuff out once, then it’s a simple matter of re-using that knowledge for other manuscripts and for new works.
I’d definitely do your learning on the Mac; you can organize your project files the same way on both platforms, but the Mac is more advanced currently (it has been around for many more years), so it will have the best set of options available for formatting things.
I’d suggest going through the video tutorials or the “Interactive Tutorial” found under Help (or in the template chooser window) before you get too deep into trying to produce a nice ebook edition out of Scrivener for the first time.