OK! I played with it some more and discovered a solution. But it required some editing.
1. I had remove any define WP_USE_THEMES references. That just botched everything.
2. I had to use ../../../wp-blog-header.php as the path in the require. For some reason, using an absolute path (with https://www…etc) caused it to fail.
Thank goodness this worked. I was beginning to lose my mind.
