Adding Tix Widget to Tkinter Container
I have just learned that apparently only root needs to be a Tix class. Since Tk, and therefore ttk, classes appear to be added to the Tix root just fine (since most of them extend the Tkinter classes anyway), this appears to be a "fix". So my problem may have been solved by changing just
root = tk.Tk()
to
root = Tix.Tk()
This did require that I pull Tix into a part of my program I wasn't wanting for encapsulation purposes, but I guess there's no other way.