How do I make wp_localize_script variables available for child theme?

In your child theme, set the priority on the wp_enqueue_scripts hook to run later than the parent theme, and do your dequeue/re-enqueue there. Right now, your child theme code dequeues the original script on a later hook than your enqueue code.

function enqueue_parent_theme_style_and_js() {
    wp_dequeue_script( 'jscript_custom' );
    wp_enqueue_script('jscript_custom', get_stylesheet_directory_uri() . '/js/jscript.custom.js', array('jquery'), null, true);
}
add_action( 'wp_enqueue_scripts', 'enqueue_parent_theme_style_and_js', 30 );