Child Theme header1.php file not overwriting parent theme’s header1.php file

That edit explains it. Alphabet is poorly written. Its use of php’s include rather than WordPress’s own include functions like get_header and get_template_part is breaking the parent/child hierarchy. You could probably make a copy of the file header.php and change those includes, as long as the file calling in header.php does it correctly!

As long as alphabet_get_option returns just those valid values or something falsy this should fix it:

<div class="header">
  <div class="inner_header">
      <!-- Start Header Position -->
      <?php
      $header_layout = alphabet_get_option('header_position');
        if( $header_layout ) {
            get_template_part( 'includes/header_layout/' . $header_layout );
        }
      ?>
      <!-- End Header Position -->
  </div>
</div>