Say10, thanks for the detailed measurements. It looks like you got 0.372mm difference. I'm sure mine was somewhere above 1mm. Unfortunately, I've thrown out the original pads. They were so ground down and dirty with caliper grease (which is completely useless, so I won't be applying that again).
I installed the spring with the arrow pointing up. After persistent problems, I tried it pointing down just for grins, but it was the same. The dealer also could not solve it, and they came to the same conclusion that the spring is not sufficiently expanded by the new pads.
So the update after a 150 miles is that the loud clicking seems to have stopped, so perhaps the spring wore down or otherwise settled in. I'll open it up and have a look once I muster the motivation.
Also, I successfully stopped the squeal again using my water technique. That is, once a week, after returning home from a ride I douse the rotor and pads with water. It took a few dousings before it was silent.
Cheers.