KB5000842 fails to install

After many months of updates and optional updates refusing to install, and eventually Microsoft store apps starting to fail too, I did a clean install of Windows, and afterwards these updates installed straight away. So for anybody who has run out of patience like I did and you don't mind doing a clean install, I highly recommend that as the way to fix this problem. 

I would caution against waiting for this to be fixed by Microsoft. Whatever is going on, it's some pretty serious corruption in Windows, because multiple updates couldn't sort this out. Don't punish yourself by waiting for a fix like I did, by the end I was really frustrated and annoyed by these updates failing and failing over and over again. I think they failed 30-50 times over the year on my PC, I manually started them many times, 8 times a month or so, and there were many times Windows tried to install them itself. 

If you're sick of this rubbish like I was, try doing a clean install. It has a much better chance of success than expecting the next update to work.