Step #4 onward was where things went wrong.
To be clear all that can be restored is the content database for the non-Administration web sites (which I think you understand) but I wanted to be sure.
Here is what you do to restore a backup of a content database to a non-Administration website.
a) Create a new web application in Central Administration and use a name like 'Temp...' for the content database.
b) Restore and attach the backup of a content database in the appropriate SQL instance.
c) In Central Administration, add the content database to the web application with the temp database
d) Delete the temp database in Central Administration
That's it. The web application will now work like it did before.