This is a good description. Just want to add a few things (or mostly reiterate/paraphrase what you said above)...Consider these two cut shots:
View attachment 285964
They both have the same cut angle, so theoretically, they are the same degree of difficulty.
However, realistically, the 2-ball is a tougher shot because your field of vision (as you're looking over the CB towards the 2B) only includes one of the two rails that intersect at the pocket, and there is no other direct visual reference as to which direction the 2B must travel in order to reach the pocket. It is my belief that this is what defines a back cut, and also explains why they are usually more difficult.
When you're shooting the 1-ball, you have the luxury of seeing both the long and short rails in your peripheral vision, as well as having the long rail as a direct reference. Assuming the 1B is frozen, you don't really even need to know where the pocket is. Just move the 1B down the rail and it will reach the pocket.
-Blake
For most shots that aren't difficult back cuts, when down on the shot you usually have one or two things in your field of vision that you could use as reference to help you locate the contact point on the OB. This could be the view of the pocketing opening, or a rail closest to the OB (as in the one ball in your above example).
But for difficult back cuts, you don't have those frame of references in your field of vision. You have to locate the OB contact point while up on the shot and remember that exact location as you go down on the shot. Once down on the shot, there is no guess and check of the contact point without significant eye movements, simply because you don't have those reference points in your field of vision. That is what makes back shots difficult. You have to locate the OB contact point while up on the shot, maintain it when going down on the shot, and trust that you still have the correct contact point when pulling the trigger.