Functions to enforce showing of excerpt regardless of previous checkbox selection, and to hide the excerpt checkbox option.

role-cleanup.php

<?php
// Update post to force show of excerpt
function edit_post_show_excerpt() {
	$user = wp_get_current_user();
	$unchecked = get_user_meta( $user->ID, 'metaboxhidden_post', true );
	if( !empty( $unchecked ) ){
  		$key = array_search( 'postexcerpt', $unchecked );
		if ( $key !== FALSE ) {
			array_splice( $unchecked, $key, 1 );
	    	update_user_meta( $user->ID, 'metaboxhidden_post', $unchecked );
		}
	}
}
add_action( 'admin_init', 'edit_post_show_excerpt', 10 );

// Remove checkbox to hide/unhide excerpt
function remove_post_excerpt_checkbox() { ?>
    <style>
      .metabox-prefs label[for="postexcerpt-hide"] { 
        display: none; 
      }
    </style>
<?php }
add_action( 'admin_head', 'remove_post_excerpt_checkbox' );