How do I write an inventory list plugin that creates dynamic details pages?

Create a custom post type inventory. You get a separate post editor, nice URLs for free. No need for a short code, because each post type has its own archive page. You may add custom meta data, taxonomies etc.

Write an importer. Look at the WordPress importer, it uses XML too. This isn’t very hard, just make sure the post type and all additional meta data are already registered.