You don't get it. Page numbers are a big mistake. For example, if you read the book using Marvin, all of your page numbers will be incorrect. If you read your book as a kepub on a Kobo Reader, all of your page numbers will be wrong. If you read this book using iBooks, your page numbers will be wrong.
The only time your page numbers would be correct if if you used ADE page numbers and read the book in ADE or a program that supports ADE style page numbers. All the other programs will not work with your page numbers. So please forget page numbers. They don't work properly.
I've seen ePub where the original ToC was left in (just as text) and all of the page numbers were wrong. You have to stop this obsession with page numbers.
Yes, use a word processor to edit your book. But if your book is not allt hat complex (formatting), you can save as text, add in the formatting when you create the eBook, It's actually easier than some of the cleaning up you might have to do from the word processor output. A good text editor (such as Notepad++) can easily help you add <p></p> to each line of text. Then you can import this into Calibre's eBook editor or Sigil, split where you need to split, add in the bolding, italics, and styles as needed. It's not hard to create a simple CSS stylesheet by hand.