“Display Name Publicly As” default setting and option

I want to have new users register themselves, but I want them to be very limited in what they can do.

In particular, I want the default setting for the “display name publicly as” option to be to “Firstname Lastname” rather than “username” which it is presently set to.

I have installed the WP User Manager plugin which generates firstname and lastname fields in the registration process, so there shouldn’t be a problem in terms of not having a first name or last name to display.

Moreover, I want to prevent new users from being able to change this setting themselves — i.e. if you are only a subscriber, you should only be able to alter your name, website, and email address, rather than how content that you contribute is displayed on the site.

I’d be grateful for any assistance. Thanks.