LOL, I had to laugh at that line...I have been thinking of this for years (even decades) now, trying to show the relationships.
And what I concluded (that you already seemed to understand) is that things shift and change so much that a political spectrum drawn up a short time ago may be good only for explaining what the spectrum appeared to be at a given point in time.
Further, it may only be good for the perspective of a specific person or group to describe a situation as it is at that time, for THEIR own purposes..
You said: "... Eventually, Hitler turned on Stalin and invaded the USSR in June of 1941. Remarkably, dedicated communists around the world found ways to rationalize these developments in supposed ideological terms..."
In one of the most influential political books (for many of us besides me) I have ever read is Whittaker Chambers's Witness. You can see how this event completely crushed him as a Communist, and contributed to his decision to flee the Communist Party.
But it clearly highlights the danger to all men: The simple fact that if we believe something hard enough are not open to introspection, new information, or simply changing our minds, then the one power that humans possess that seems to be second to none can kick in and lay waste to our intellect: the power of Rationalization. We can make nearly anything fit what we want it to fit. I am guilty of it and have to guard against it, and even still, I fall prey to it.
Unfortunately, many people don't even see that they must be on guard against inappropriate rationalization.
Because as humans, we are damned good at it.
As for ideological categorizations, they tend to be bipolar because that is how politics works. Those in power are contested by those out of power, with ideas, policies, resources, and people sorting out on that basis. Although those elements can also be organized in other ways, the struggle for power matters most.