Well, none of the above worked, but I did turn the JQ datepicker off. With it off, I was able to enter any data format I needed, and it worked fine. I narrowed the problem to this function in db.php:
On about line 348 (I entered a couple comments above), the code reads:
$chars = str_split($format);
It very strictly parses the data based on the specific format, and fails the date if it does not exactly match, even if it is a valid date. This code seems unnecessary as, with the JQ_datepicker, a valid date format is forced on entry and the date must be converted to 'Y-m-d' for saving into the database anyway. I bypassed the function and used only date_parse(), and everything worked perfectly. :)
Line 39 of admin-main.php contributed to the failure as well. It required a date format that was not set by the date settings and differed from the format entered by JQ_datepicker. I changed that format to match what JQdp forced, and the code worked fine then, too.
It seems to me, though, that the date_parse_from_format() function is entirely unnecessary, as you are not returning a date with a specific format that users must view. You are simply validating the date. Another function returns a formatted date for display, based on data pulled from the db table.
Those are my thoughts, anyway. Hope that helps. :)