To set configuration at the module installation

Exemple:
$site_name = \Drupal::config('system.site')->get('name');
// Set the site name.
\Drupal::configFactory()->getEditable('system.site')->set('name', 'My site')->save();

You can do that using Symfony JsonResponse.

Exemple 1.
return new \Symfony\Component\HttpFoundation\JsonResponse($array);

As the solution depend on your error, first of all, you must find why drupal crash.

1. First thing is, You must check the serveur error log.
# tail -f /var/log/apache2/error.log

Exemple:

use Drupal\Core\Url;

// Internal path (defined by a route in Drupal 8).
$url = Url::fromRoute('help.page.views');
$internal_link = \Drupal::l(t('Views Help Page'), $url);