Changing to date field based on from date using javascript
If I understand you right it should be as simple as dropping something like this into a JS file included in the page:

(function($) {
  Drupal.behaviors.myModule = {
    attach: function(context, settings) {
      $('#edit-from-date', context).change(function() {
        $('#edit-to-date', context).val($(this).val());
      });
    }
  }
})(jQuery);


Disabling certain dates is a bit more complicated but there's an example here that should point you in the right direction.