1. I have chosen one of your single product page templates as the new default product page template
2. Creating a new product page with this new template works fine, but when I later edit that product using Elementor Pro and save my changes, it switches back to the original theme default template.
Do you mean Woocommerce builder of WoodMart allowing to create custom templates for product pages and other functional pages that are not editable by default. Woocommerce builder is the inbuilt functionality widget included in the builder you use on the site.