Expose Site context via REST with Community Guest User Access

Having a community site and planning to build mobile apps. Is it possible to have the guest user authentication from a mobile client? Below are the concern.

1. Community user registration need to be done via custom REST Services
   which uses Site.createPortalUser(), but Site context is not
   there when you connect the REST service with normal user
   authentication. (Site.Login() etc. also wont work I think)

2. Is there a same authentication mechanism like in browser for REST,
   where public pages are accessible with guest user profile and
   whenever the actual user login in session will be transferred to
   that user.

There is an option on the Guest User Profile as API Enabled. Not sure if this will be helpful here. In Developer Edition this feature is not there to give it a try.

UPDATE :
for portal user creation and all I need to use my own apex classes REST end points. So to access them it needs authorized access from mobile client AFAIK. But if I use normal user to authorize, Site context is not in scope.