The tooltip for coupling/decoupling the instance unhelpfully does not change no matter whether the button is activated or deactivated (i.e., lit or unlit). Therefore, I'm unable to determine which state the instance is in. My search in the manual, the forum, YouTube, and other resources has not resulted in the answer. So, here I am, asking a remarkably basic question: in which state is the button to decouple an instance? Lit or unlit?

In my experience, when the button on the plugin is LIT (orange) (or the little RED button to the right of that is LIT RED), that means you are UN-coupled.  The orange button un-couples only THAT instance while the little RED button un-couples ALL VEP instances.  If both of them appear gray (UN-lit/colored), that means they are coupled.