1. I'm not sure how a password manager is related to a site owner who is trying to create a single database of his multiple sites users, which lists the users and the sites without duplicates.
Might want to try KeePass which is open source, free, and secure.
http://keepass.info/
I would never use Lastpass, they were hacked once in 2015.
2. If the username was already taken then the new user wouldn't be able to signup using the username. But, you do raise a legit question if the module was installed by someone who already had multiple JR sites with users who signed up on both sites. There would need to be some kind of checking feature of some sort. The admin would have to determine how to handle two different users who were using the same username. I'd admit, that would be an issue.
3. You would only be reserving your name on the JR sites that one person owns, not every JR site in existence. If I owned 3 JR sites, your name would be reserved on my three sites if you signed up for 1 of the JR sites.
Maybe unified Login was a bad choice of wording. How about unified JR Login for a particular owners sites. I would install the cloud module and it would run on my JR sites and only my JR sites.
Flowchart (using X corporation as the owner of several JR sites):
You would come to one of the JR sites and be presented with 2 signup options.
A. Normal Signup for someone who never registered on any of X corporation's sites.
B. Selection asking if you're already a member of any of the X corporations JR sites? If so, enter your username and password from any of those JR sites.
If the user picked A it would:
1. Check if that information was already populated in the master cloud database and if it found a match, return that the user was already a member of one of the X corporation's JR sites and:
a. Ask the user to enter new / different information because the entered information already exists or
b. Enter their username / password from the X corporation's site at which point the existing data would be pulled down from the cloud server.
2. If there wasn't a match, create a new account for the user on the JR site and upload the basic information about that user to the cloud database which would reserve that username for all of the X corporation's JR sites.
If the user picked B it would:
1. Pull down basic information about the user and populate that information in the local JR site's database and mark in the cloud database that he was now a member of that site along with the other X corporation's JR sites.
This would be great for targeted marketing if I had 4 JR sites and a member who signed up on 3 of those sites. I could target market to that user and hopefully get them to signup on the 4th site being that they were already signed up on 3 out of 4 sites.