RE: Registering New Users
Testing creation of new WP username only accounts has been plaguing me, too. And many others, lately, seem to have the same problem, resulting in new users not showing up in the Users list (and not landing on the page with their dashboard).
Here's what I discovered:
Sometimes the the link for "sign up for just a username" doesn't work; when clicked the page it is on just refreshes without change, whereas it should refresh whith the Blog Address input box (and other things). The problem is related to cookies for WordPress (and/or the page the Register link is on).
Clearing cookies resolves the problem (temporarily), but eventually the problem reoccurs.
So this is not a fix, but rather a clue as to the root of the problem which WordPress needs to resolve.
Another problem I have is with Register on the Meta widget. Since most new users my website--and I imagine many other websites--need to register for a username only. So why not give them the correct page to start with, rather than having them notice and click the link ( "sign up for just a username") that creates the correct page for them.
Let's make Register naive user-friendly.
My workaround was to use the replace the Meta widget with a Text widget that has the correct link for Register. Unfortunately, it doesn't fix the cookie problem, above. WordPress really needs to fix that real soon, now.