Firstly, your backup solution is somewhat unusual. The recommended method is using Time Machine, or either Carbon Copy Cloner or Super Duper to make bootable clones. Time Machine has the advantage of making access to the backed up files easy should you find that you have deleted something by mistake. With CCC and SD, you can boot your Mac from them should your internal drive fail.
Second, you really don't need an antivirus app. There are no viruses for Macs out there and you can be sure that when the first one appears it will be all over the news.
It was some time later that I realised that my user account changes were probably the reason for the MBA’s behaviour changes and that something had not been corrected.
Yes, that may well be the case.
First, what changes and where do I need to make them to restore USER to the full administrator access that it had when the MBA was new?
To be honest, I couldn't say. You may have to do a reinstall and start again.
how do I create a second (standard) account for Internet access that has access to all the applications, folders and documents that USER has?
You can't. A Standard account has access to the main Applications Folder(same as an Admin account), but can only access folders and documents in it's own Home Folder.
If the Admin account wants the Standard account to have access to Admin's files/documents, Admin must put them in the computer's Shared folder or his Public Folder. However, Standard account will only be able to read them, not edit them.