Sample code for validating custom metabox?

Straight from the WP Codex @ http://codex.wordpress.org/Function_Reference/add_meta_box, you call the save_post hook and specify the function that will be run to validate/save your data:

/* Do something with the data entered */
add_action('save_post', 'myplugin_save_postdata');

Then you define that function, which will automatically be passed the post id. Additionally, you can access the $_POST array to get the values in your metaboxes:

/* When the post is saved, saves our custom data */
function myplugin_save_postdata( $post_id ) {
  // verify if this is an auto save routine. 
  // If it is our form has not been submitted, so we dont want to do anything
  if ( defined('DOING_AUTOSAVE') && DOING_AUTOSAVE ) 
      return $post_id;

  // verify this came from the our screen and with proper authorization,
  // because save_post can be triggered at other times

  if ( !wp_verify_nonce( $_POST['myplugin_noncename'], plugin_basename(__FILE__) ) )
      return $post_id;


  // Check permissions
  if ( 'page' == $_POST['post_type'] ) 
  {
    if ( !current_user_can( 'edit_page', $post_id ) )
        return $post_id;
  }
  else
  {
    if ( !current_user_can( 'edit_post', $post_id ) )
        return $post_id;
  }

  // OK, we're authenticated: we need to find and save the data

  $mydata = $_POST['myplugin_new_field'];

  // Do something with $mydata 
  // probably using add_post_meta(), update_post_meta(), or 
  // a custom table (see Further Reading section below)

   return $mydata;
}

All of your routines to valid data will be done within this function. In the end, you will likely save the data using something like:
update_post_meta('meta_key', 'meta_value');

EDIT: I realize I did not address the breadth of your question, but since I put the time into this I leave it here to get you a quarter of the way there.