Thanks for reaching out.
Blog index where post listing is displayed is a reserved Wordpress page. It can't be edited by any builder other than template customization. And I checked your sites and I don't see any blog, perhaps you haven't created a blog page that Wordpress can use for listing.
In that case, please check this https://www.wpbeginner.com/wp-tutorials/how-to-create-a-separate-page-for-blog-posts-in-wordpress/ (skip the page template, it's automatic in X/Pro theme). And setup your first Blog index page, again, it's a reserved page so it can't be edited through the builders, even Gutenberg (Wordpress' own builder) can't do it.
This is Wordpress limitation and restriction and has no connection with Pro features. But if you wish to create a custom listing (but not actually a blog listing), you can then use the Essential Grid or The Grid plugin for displaying recent posts to your preferred pages.
As for your two questions,
1) It always displays the home page, unless you already on the blog page (front site) and hit
Theme Options from the admin bar, then it will display the Blog page (if you already have an existing blog page).
2) It's not possible, each stack has unique features and can't be mixed together. But yes, you could do a sub-domain (separate installation or a multisite setup) and have a different stack. And since it's just the same site/project and just separating the sections then you don't need a separate license.