Update: 2 hours later it's still spinning.
I've been told it's a slow install. Did not expect it to be this slow.
If it's still doing this in the morning, I will likely have to implement Plan B.
(I finished Plan B, reformatting and reinstalling Manjaro on an SD card, while still waiting for Plan A to finish.)
Plan C is an old touchscreen EeePC with a new battery that also happens to have Manjaro on it.
Update: Having allowed it to churn all night, no progress has been made at all. I'm forced to assume that the emmc installer wasn't actually doing anything as the emmc remains untouched.
Fortunately plans B and C are go for launch.