1. The search feature is missing posts--how do I ensure all posts are being searched (the query is correct)?
2. How do you use class "ari-fancybox-iframe" so that if the featured image is selected the linked post appears in a lightbox?
Welcome to our forum.
#1, Post grid search use default parameter "s" and by default, not that advance search result, there is a filter hook for query arguments, you could take advantage to make advance search form and search thought Post Grid, please see the documentation
#2, not for thumbnail, you can add custom class to item wrapper by filter hook, please see the documentation
There is also HTML element under layout editor, you can pass any shortcode to generate custom HTML each item(post) of post grid.
Let me know if you understand correctly.