I’m marking this as resolved and moving the additional issue to a new topic.
I checked with phpMyAdmin and found only the entry related to the plugin in the database (I changed the error message and saw the change reflected in the table). The mystery version is not found by searching the database. However, I can see it even after clearing site cache, accessing with another browser, etc. How in the world is this showing up? How can I get it so that only the legitimate, intended checkbox shows instead of the phantom one?
Switched to classic. Now it is showing up twice on the checkout page. 🙁
