I tested preloading much more thoroughly this morning. There clearly are serious problems with several aspects of way it works that are compounded by confusing and misleading text displayed on settings pages. For example, based on the way the software actually seems to work "Scheduled preloading of cache in 10 seconds" should instead say something like "Cache preloading will begin when the first website visitor attempts to load a non-cached page 10-seconds or more after you clicked the 'Preload Cache Now' button."
Requests to load a page that is already cached do not start preloading under any combination of settings or conditions I have been able to find, but attempts to load a non-cached page will start the process after 10-seconds have elapsed, even if preloading is not selected. However, immediately after all website pages have been cached, all but the home page and one other random page are always deleted from cache regardless of optional settings. The second page that remains cached with the home page can be any website page and changes each time the process is repeated.
Sometimes preloading gets locked in a loop that causes PHP to timeout, even if PHP's max_execution_time is set to several minutes. After a PHP timeout there is a long delay before Apache can be stopped and restarted to clear the resulting error condition.
Preloading worked in prior versions, but there are serious problems with it in the current release. I have temporarily disabled WP Super Cache at all the sites where I have it installed because of that.