Well, now I'm screwed. Based on your implications, I attempted to eliminate the accused - namely the template - by switching to the original, 'Classic Contemporary Green' template that comes with Zen Cart. Now I've essentially got NO template:
https://store.ozurojewelry.com

Here's the deal, if I try to make a 'subtractive' change within the admin panel -meaning, a change that first removes a setting/value and replaces it with another setting/value,) the first half works (the setting/value is removed,) however the second half doesn't work - the setting/value isn't repopulated. Please understand, this is just my logical analysis of the issue, as I'm trying to understand what's actually happening, and I'm hoping said analysis will help one of you to figure out what's wrong.
As indicated in my initial post (though admittedly, my current angle is based on 20/20 hindsight,) any time I tried to ACTIVATE a side box (which would be an addititive change, thankfully,) the box wasn't activated. Things continued on as they had before without any change. However, now we know that if I try to CHANGE a setting, such as what template I'd like to use, it appears to successfully deactivate the current template, but can't activate the new one.
Here's another example - I just went into the Admin/Configuration/Website Maintenance panel. "Display website maintenance period" was set to 'FALSE'. I clicked it, clicked 'Edit', changed the value to 'TRUE', and now there's no value at all:

Seems to me that should help someone experienced diagnose what's wrong..hopefully...it's just...a file...with bad permissions... At least I hope.
:frusty:
-Dan