On the drive in to work I was thinking about the rolling gear and the discontinuity at the edges of the canvas.
It struck me that instead of messing around with starting and stopping things, maybe what I should be doing is reversing the sense on the gear rotation, and simply move the animation right to left.
If I do that, I can lose the entire reverse canvas, the outer Storyboard becomes a RepeatForever, and I can just time the right and left animations.
Not sure when I'll have time to try that, but questions I have are with regard to the time it will take to swap those settings at the canvas edge.
If it works, it certainly makes the whole thing cleaner.