Slidemaster associated with scene - can't delete

I've been having trouble identifying an old slidemaster layout using the usual tricks (rename slidemaster to "TO DELETE", using bold colour shapes). I just discovered that the slide layouts I'm searching for aren't actually being used on a slide, but are attached to the scene and preventing me from deleting from the Slidemaster. 

In the attached image, you'll notice that slidemaster TO DELETE is in use for the highlighted scene. Every slide in the scene is confirmed to not have this layout. Not one single slide in this course.  

Further, when I create new scene to try to start fresh, it defaults to selecting the layout from TO DELETE which I'm trying to remove. 

Is there a way to remove the scene association with this slidemaster? And, is there a way to change the default slide layout to the slidemaster I intend to use moving forward? 

Thank you