Is it possible to return a form element AND an ajax response from the same ajax callback function?
If you want to do more than simply return new markup, then your response should be an instance of Drupal\Core\Ajax\AjaxResponse, to which you can append one or more AJAX commands via the addCommand() method.
A simplified version of this would be:
use Drupal\Core\Ajax\AjaxResponse;
$response = new AjaxResponse();
$response-&gt;addCommand(new ReplaceCommand('#my-wrapper-id', $form['fid']));
return $response;

I recommend checking out a tutorial, such as one of those below, for more information on using AJAX with FAPI:

https://www.sitepoint.com/using-ajax-forms-drupal-8/
https://www.drupal.org/docs/drupal-apis/ajax-api/core-ajax-callback-commands