I don’t know how many more of these failed-update customers we are going to get but wouldn’t it make sense for the instructions in post #14 just to include the sudo ?
Where it is unnecessary, it should be harmless. Where it is necessary, it should avoid a lot of grief for already-flustered customers.
Big disclaimers apply as I haven’t looked at any code but:
The procedure with the volume button puts the Librem 5 in “serial downloader” mode. (lsusb output on the host will be different, as compared with a normally booted Librem 5 that is plugged into a host via USB.)
The procedure with the battery is not usually relevant or necessary but is sometimes needed in obscure cases, according to @dos.
In serial downloader mode, commands and data can be sent by a USB-connected host to the Librem 5. That of course needs some client software on the host. That client software is uuu
uuu is capable of many things but one of those things is transferring a bootable image over USB and having the Librem 5 boot it. One such bootable image is “Jumpdrive”.
“Jumpdrive” configures the USB device at the Librem 5 end to advertise one (or two) USB mass storage class devices (disks) and some kind of network device (sorry, details not in front of me). If you have a uSD disk inserted then it will advertise two disks, otherwise just the one (the eMMC drive). Yes, it configures a network interface and starts a telnet daemon.
(Handling of partitions is just standard functionality on the host.)
From having watched the build, for 5 hours 
, the Jumpdrive kernel is fairly comprehensive. Once you telnet in, you would expect most things to work normally. So you should be able to mount the disk(s) locally - if you didn’t mount them on the host - and wander around the directory tree. However I can’t give an account of what things are missing and what things are included.