* Can you please clarify which ‘Trending Stories’ section you are talking about? There is no Trending Posts section which automatically shows the most viewed posts.
* You can change the footer featured posts to latest posts as well as any category and any post number you like just go to Customizer > Footer Featured Posts there you can set the posts type you like to set.
Yes, Trending Stories section relies on a category rather than on views.
For to set the category posts for footer featured posts just see this video tutorial.
Yes the demo data was set to whole from the latest posts because of category id which will miss match with customer site. You have to set the category for each and every sections individually. You can see this video tutorial for to set category from different sections manually.