A couple remarks

Nice work, and interesting use case for remote data !

A couple remarks :
- you probably want to add a dedicated index on 'rating' for sorting in Views

- ctr_field_text_widget_validate() :
You can probably replace this :

$delta = $element['#delta']; // TODO: Isn't there a better way to find out which element?
$field = $form_state['field'][$element['#field_name']][$element['#language']]['field'];
$field_name = $field['field_name'];

if (isset($form_state['values'][$field_name][$element['#language']][$delta])) {
  $values = $form_state['values'][$field_name][$element['#language']][$delta]; 
  ...

with

  $value = $element['#value'];

- About refreshing on cron : would probably be easier if the 'update_on_cron' setting was field-level instead of instance-level (plus I'm not sure there's an actual use case)