Okay, I've got scheduled posts up and running now. I read this four-year-old message thread v. carefully:
Scheduled Posts Still Not Working
Before I added the new callback function recommended by Samuel Wood in that message thread, I also cleared out any plugins that might conflict with post scheduling, did a check on my theme for same, and then combed through my wp-config file looking for any code debris left by plugins I was no longer using. (A caching plugin I tried and removed two months ago left a few things laying around in wp-config that probably should not have been there.) And finally, I reinstalled WordPress for good measure. (Some of these checks were recommended by W.ORG Moderator Esmi in that same four-year old thread.)
At that point, I added the callback function mentioned in the message thread above.
I have run four staggered test posts for the last two hours, and posts now seem to be posting on time. However, my grandmother used to always say, "There's no such thing as a free lunch," and boy, was she right. Please read the warnings in the next two paragraphs.
A NOTE OF WARNING: After a year of heavy studying, I do understand why the added define parameter works, but just barely. Much of my understanding of this fix today has to do with the person who wrote the fix, Samuel Wood. Please do not ask me for further explanation if you do not understand the link above, because I will be useless. Seek out a proper volunteer moderator and ask her/him for help.
CAVEAT EMPTOR!: One more thing, and this is important to understand before you do anything: Wood clearly states at the end of his explanation that this method is not an absolute fix but a workaround, and this workaround may not have consistent results.
FINAL THOUGHTS: If I find more than a casual inconsistency in posting, I will come back and mention it here or I will open a new thread where I will ask a proper moderator for assistance. In any case, I live in hope. Best of luck to you all.
EDITED ONCE: For clarification of a very muddy sentence.