OK, so I admitted defeat and ordered a second hand part from a QLD-based wrecker. Double-checked via email that it was quiet, no noise, and working as it should, and pulled the trigger.
Arrived yesterday, took to auto sparkie today.
It was different to the one in the car.
Same overall shape, squirrel cage same diam, same height, but motor casing at rear wider and taller.
Approx 7mm wider and 10mm taller.
The plug to connect the blower on the harness was also smaller and a different shape to the socket in back of the motor casing.
We hot wired it using some copper connectors the sparkie had in stock, to test it, and it was fine. Probs 20% quieter than the old one. Which bizarrely was dead quiet (to naked ear) when powered up in footwell, but noisy and rumbling when installed under dash. Go figure. We tried moving things about under the dash but there was no sign of anything rubbing or whatever. Just weird.
Checked the 'new' one as quiet in footwell, and then installed temporarily in dash, still quiet.
So sparkie spliced into harness above OEM plug (which now doesn't fit) and wired in a set of connectors that would fit the male pins on the 'new' blower. Re-tested. All good. Everything fits. Though there is almost zero clearance between blower motor and undertray.
[We left the OEM plug in situ in case I ever need to swap back to the 'genuine matching' motor, which I kept as a spare.]
Sparkie charged me AU$60 for half hours work (though it took him an hour).
Wrecker charged AU$95 for part, freight included.
$155 vs $525 from Subaru + sparkie to fit.
Calling that a win.