Theme won't allow featued image from media library

My theme has a built-in image attachment function that prevents me from setting a featured image from an existing image in the media library.

The function is based on a recipe called Image Attachments Toolbox, which allows for auto creation of a gallery by uploading multiple images while in the post editor. This is a cool feature, but the drawback is that the only way to register a featured image is to upload and set it from within the post editor, so it can be pretty time consuming when publishing 50+ posts a day.

The theme is WP Navigator (premium theme from Themeforest).

Here’s the code snippet that registers this feature in my function.php: https://pastebin.com/3M8uKYxR, and here’s the hook in my single.php that executes the function: https://pastebin.com/U7VBfixG.

My goal is not to remove the auto-gallery feature from the theme, but rather to be able to bulk upload multiple images to my library and select one for each individual post’s featured image before publishing.

This has been nagging at me for a while so I’m really hoping someone can help shed some light on how to resolve this issue.

Thanks in advance for any suggestions!