Wow, quite a lot of questions. I'll try to answer a few:
As long as the documents (QVWs) don't use new features provided by the QV12 release, users of older clients (either v10 IEPlugin or v10 Desktop won't notice the difference. Except for the thousands of bugs that have been fixed between v10 and v12.
Best practice dictates that you should upgrade all software components to their v12 counterparts as quickly as possible. Start by upgrading the servers. Since the IEPlugin cannot be had as a separate download anymore, you would need to install the v12 servers anyway before you can get hold of the new IEPlugin installer.
I think you can still use the QlikView Upgrade & Migration Document. See Attachment.