OneNote - export all sections in a notebook to Word? Or other method for same result

The first alternative, with better results, is to use One Note PowerShell with the explanation given in this blog to export all pages changed after a specific date (hum... let's say... pages created after 01-01-1970) as one single .mht file, then edit that file in your Word.

There's also the OneNote Power Toys that can be used to merge pages in One Note...

The OneNote to HTML Server allows you to save one html for each page that is changed in a One Note directory, and if you change them all, you'll have a one-html-per-page. The problem is to make a little change in every page.

And, finally, the One Note Web Exporter creates a web page that you can use inside any browser. But it won't be a single, continous HTML, it will resamble your OneNote page-separation. Perhaps easy to edit the resulting web-page to make it a single html, but I'm not sure.