For "Document is licensed for a different user account" in the middle of your troubleshooting you most likely changed the DRM account on your computer. Opening a support case ( http://www.adobe.com/support/digitaleditions - click on 'Submit a web case') will eventually get you to a tech who will be able to look at the books in question and tell you what the AdobeID was that you originally purchased content with was.
If there was no AdobeID (because you were anonymously activated) they will be able to create an AdobeID for you and associate that with your anonymous activation.
However be prepared for a long wait, because this will not be a "simple" tech support case such as an activation reset (which currently take 3-5 days)
Also if you content purchsed with two different DRM accounts (ie AdobeIDs/anoymous activation) which certainly sounds like the case, you will not be able to merge the content, so you will end up having to decide at any one time which content you would prefer to view. So I would suggest that you purchase as little as new content as possible until you get this resolved.
An aside for everybody reading this.... DO NOT ANONYMOUSLY ACTIVATE YOUR COPY OF ADE. This will lead to heartache and trouble, in the form of either losing your books in frustration of dealining with Adobe support.
Thanks so much Jim for your prompt and helpful answer. I've sent a web case as suggested and we'll see what tech support can do.
Many thanks again (and I shall not be authorising my ADE anonymously!