I've tried to reproduce your problem(s). I've run into the issue that the password locking isn't set properly for some reason in the past, but as you've found out yourself that is easy to circumvent.
The other issue ("the username is already in use") is a bit tricky, it seems. I've been successful in triggering it exactly once, but I haven't been able to reproduce it afterwards – no matter what I tried. I'm therefore thinking this is a corner case that's not handled properly by the backend. When you create a new user the UMC creates a temporary user object, sets it up properly, and only after everything has been done it moves that temporary object to its final location. If such a temporary object is left in the tree this error message may be triggered.
That one time I did trigger the error ("the username is already in use") I could click "save" a second time, and that time the user object was saved successfully. I therefore recommend you try that, too, if it happens again.