How do I prevent the theme from using single-fullwidth.php as _wp_page_template when creating posts? I want it set to default so that an Elementor template is used. When I try to set Elementor global settings for page layout, it does not have a default option in the dropdown like there is when editing a post. When using theme default, it defaults to single-fullwidth.php instead of default. As a work-around, I have hooked into wp_insert_post to update the post meta and set this meta key to default, but wondering if there’s a setting I’m missing somewhere to have it work this way by default.

Thanks for the information, but unfortunately I do not make changes to core theme/plugin files since they will be overwritten in updates. I am already hooking into the same action to accomplish what I need. The theme just doesn't seem to be fully compatible with Elementor without this change. Since you're recommending a core file change, I'm assuming there is no setting to make it work as expected. 

You can reproduce by creating a Theme Builder template with a condition to apply it to all posts and see that it never gets applied unless you manually edit the post settings in Elementor and change the page layout to default.