After much teeth-gnashing, I realised I had made an error with my regex transformation. I rectified this to no avail.
I thought harder, and broke up the strip into 4 sections, left, front, right, rear in the code. There are a total of 750 LEDs.
I reordered the sections to be left, rear, right and front. Saved the .json file and uploaded it.
This time, the front and rear HAD been swapped, but now the front and rear have been flipped, so left is right and right is left. Getting closer!
I then displayed the configuration in Hyperion LED instance, and noticed, as it opened, that “reverse” got selected and the LED image on my browser flipped.
NOW, the front and back are exactly the same, while the sides are all lit the same as the first and lest LEDs respectively. It’s OK for now, though not what I want.