It is actually possible to resolve the entire thing without another computer or even without using a USB stick. Or at least, it was for me - I did briefly need my phone, but that was due to an additional problem which not everyone may encounter.
What I did (after many hours of searching online for solutions) is as follows:
- Login to your account on the corrupted computer (as per Vlk's steps 1-5), but you don't need to click away the error messages (just Alt-Tab them)
- Hit Ctrl Alt Del, then open Task Manager
- Go to Users tab, right click your account, select Manage User Accounts (If you don't see any accounts listed, try signing out and in again)
- This will open a Control Panel window, where you click the arrow pointing up until you get to an Explorer window, from where you should be able to open a web browser, access your downloads folder, etc.
- Then download the update as per steps 7-9, but select the ISO version, download and save directly onto your computer.
- Mount the ISO (right clicking on the downloaded .iso file should give you a 'Mount' option), then run setup.exe
I had the additional problem that the Media Creation tool wouldn't work on my computer, blocking me from downloading the update. I finally found an article (
https://www.askvg.com/tip-get-direct-download-links-of-windows-10-iso-from-microsoft/ ) explaining that there is actually a way to bypass the tool entirely and directly download the ISO update, but you can only access the direct download link if you open it from an OS that isn't Windows 10. Any phone with internet access should do the job, and will allow you to email yourself the direct download link, which you can then use from your affected Windows 10 computer to start downloading the file. Once it's on the computer, mount it and run setup.exe .