How to add dynamically the main parent pages’s custom fields and their values to all sub-pages?

The parent post ID is stored in $post->post_parent. So you can access the parent data by using that ID in get_post_meta().

To get the ancestors use get_post_ancestors( $post->ID ). It returns an array of parent IDs, the last one is the highest. Let’s invent a helper function for the next examples:

if ( ! function_exists( 'get_top_ancestor' ) )
{
    function get_top_ancestor( $post_id )
    {
        $ancestors = get_post_ancestors( $post_id );
        return empty ( $ancestors ) ? $post_id : end( $ancestors );
    }
}

Now we can use that helper in a filter for the_content:

add_filter( 'the_content', 'wpse_78325_parent_meta' );

function wpse_78325_parent_meta( $content )
{
    global $post;
    if ( ! is_page() or empty ( $post->post_parent ) )
        return $content;

    $top_id = get_top_ancestor( $post->ID );

    if ( ! $data = get_post_meta( $top_id, 'demo_data', TRUE ) )
        return $content;

    $extra = sprintf(
        '<p>Meta data <code>demo_data</code> from <a href="https://wordpress.stackexchange.com/questions/78325/%1$s">parent post</a>:</p>
        <pre>%2$s</pre>',
        get_permalink( $top_id ),
        esc_html( $data )
    );

    return $extra . $content;
}

Let’s say you have a parent page Privacy Policy with a custom field demo_data …

… and a child page Security …

… then the code above would produce this result:

---

In reply to your comment: You can use the parent post meta wherever you need it. In your case, I would filter wp_nav_menu_args and call wp_nav_menu with a static string for menu.

Sample code, not tested, just a draft. 🙂

add_filter( 'wp_nav_menu_args', 'wpse_78325_parent_menu_name' );

function wpse_78325_parent_menu_name( $args )
{
    if ( 'primary' !== $args['theme_location'] or ! is_page() )
        return $args;

    global $post;

    $top_id = get_top_ancestor( $post->ID );

    /* prepend this line with a # to switch the logic
    if ( ! $name = get_post_meta( $top_id, 'MenuName', TRUE ) )
        return $args;

    $args['menu'] = $name;
    /*/
    foreach ( $args as $key => $value ) // you can use custom keys here
    {
        if ( $new = get_post_meta( $top_id, 'MenuName', TRUE ) )
        {
            $args[ $key ] = $new;
            unset ( $new );
        }
    }
    /**/    

    return $args;
}