Problem with security update to 10.11.6

I tried to instal the security update issued for 10.11.6 on my 27" iMac (late 2012).

When I went to log back in, I got the spinning beachball (for many hours).

After talking to the Apple people (by phone) it appears that the update has corrupted only that particular username (so the update was successfully installed, it simply killed the user). We could figure this because ether was another (never used) User with administrator privileges, and when I logged in thru that everything was fine - except it had none of my stuff in it.

The only solution the Apple guy could suggest was to make a new User and drag everything across - unfortunately this loses me all sorts of things - from time machine not being able to operate or be accessed, passwords stored in the Safari keychain aren't accessible, my (Parallels) Windows 10 (seemingly I can't replace it without paying for a new version of W10!!), not allowing me to simply drag items stored in my computer linked to box.com (or rather I can copy them over but if I ever try to change any of the files it asks for permission and a password, and I can't assign the ownership to the new identity because that is greyed out). All in all an utter disaster so my iMac is a mess!!!!

Help, please!