HAPPY DANCE! I think I found the problem!!! It has nothing to do with variable products… it’s image sizes! The Generate Page Header plugin has a problem with any images over a certain size. Those images stopped appearing in my media library whenever the plugin was activated, and any products that had a featured image that was too large would break. Any portion of a given page that loaded after the broken product (front end or back end) simply wouldn’t load… including the wp-admin bar when logged in, which renders after all the rest of the page has loaded!!!
So, I will simply deactivate the plugin, delete my stupidly-large images and replace them with reasonably-sized images, and I think all will be well on my end 🙂
All the best, and thanks for your help in diagnosing the issue!