Does anyone know when Anna and Elsa's official coronation is?

There's no official information of it, but possibly after 1 ano anniversary of their film. Maybe at the end of 2014.

Not anytime soon. Some people from disney said there are no current plans for their coronation, because they want to focus on seperate Frozen - Uma Aventura Congelante merchandise right now, before they will add them to disney Princess line-up. At this point it is even unknown if Elsa will be added after all, since disney said when they were asked about that 'she is the Queen'.