Mini Help needed for bricked Kobo Mini

Hi there, and thank you in advance for your help!

Both my son and I have Kobo Minis.

Mine was working perfectly running 3.19.5761-s until today, when I decided to drop the Kobo Glo 4.35.20400 update file into its .kobo folder.

The result was the dreaded three vertical dots - see attached photograph. I've tried a soft reset (power button) and a hard reset (using the hole at the back of the device). Neither worked.

I'm desperate to get it working again - I take my Mini everywhere with me.

I've taken out the 4Gb SD card and I can see the contents of it on my Mac. I've tried copying the following hidden folders/file from my son's Mini to my non-working one. Sadly, it didn't help.

Folders:
.adobe-digital-editions
.fseventsd
.kobo
.kobo-images
.Spotlight-V100

File:
ddriveinfo.calibre

I was pretty comfortable flashing ROMs and using hex editors on Android devices and ereaders many years ago, but I can't remember any of it now, so please explain it like I'm 5.

What, if anything, can I do to restore the non-working Kobo?

Thank you so much in advance for your advice/support/guidance.

Attached Thumbnails