This was resolved by emailing Google. It was the www in my domain name in the Javascript Origins field. Even though that field example shows a full domain as http://wwww. when I removed the www out of frustration, it worked perfectly.
Not sure why that is, but it’s the only change I made this morning. I’m marking this closed.
I’ve been through all the posts I could find on the subject and my domain is in the Authorized Javascript Originals Field. I’ve verified my domain. I’ve enabled the Drive and Picker API, I’ve copied and re-copied my Client ID, I’ve done everything but stand on my head.
