Well the story from Subaru is:
The replacement drive-shaft that came from their supplier was a bit longer than it should have been, so the hub nut tightened down on to the end of the thread rather than pressing onto the bearing, allowing the bearing to float around.
This chewed out the spline on the end of the drive shaft and inside the hub, so today they installed a second new drive-shaft, and also completely replaced the whole hub assembly.
Here is the shaft that was installed last Thursday, that was taken out again today.

- Img_4032B.jpg (73.97 KiB) Viewed 5790 times
... and would you believe, I heard a metal-on-metal scraping noise from the same wheel while driving home tonight. Took the wheel off yet again, and discovered the bottom edge of the metal shield behind the brake disk was rubbing on the disk. Just flexing it away a bit was enough to stop the noise.