Failed High Sierra update, can't mount drives, how to recover files? The issue is with Apple MacBook Air "Core i5" 1.6 13" (Early 2015).

It had not been backed up in a while but an update to High Sierra was attempted. The update failed and now a regular boot or safe mode boot leads to the "question mark folder". Booting in recovery mode or web-recovery, then attempting to update the OS leads to "Downloading installer information to the target volume failed."

At this point the main concern is to recover the files. In terminal I can see that the files are there, but I can't mount an external drive to copy them. There is power at the USB ports but nothing shows up in Disk Utility ( or "diskutil list").

I made a bootable USB with El Capitan on it, and when I boot from that, the USB ports work. But now the SSD won't mount, so I can't recover anything. 
Running "diskutil list" shows the SSD as disk0/disk0s1/disk0s2, but only disk0/disk0s1 can be mounted (of course disk0s2 is the partition I need to access). disk0s2 is APSF, and I don't think its encrypted.

How else can I get one of these disks mounted?
The problem, whatever it was exactly, was solved by having the update completed at an Apple store, connected directly to an update server. Hopefully somebody finds this useful.