Update the defaults
Drop the options for new_user_override and match_by_email -- hardocde them to false and true, respectively, but permit customization through defines in wp-config.php or the like.
Drop the link from the settings section title, and move the links from after the input fields, up into the summary.  Props @lessbloat for the copy.
Permit calling get_edit_app_url with no parameter.
Add an admin notice if there are no credentials.
Tidy the error message displayed if it can't log in a user depending on whether or not it can match based on email.