Getting the title of the parent page and displaying it only for certain parent pages

You’re most of the way there with your current code – you’re correct that you can wrap the condition around what you’ve already got. The is_page() function allows you to check for an array of pages, so as long as you plan to specify pages manually, all you need to do is

<?php if (is_page(array(11, 'about', 'Contact Us')) ) : ?>
    <p><strong><?php echo get_the_title( $post->post_parent ); ?></strong></p>
<?php endif; ?>

is_page() accepts page ID, slug (from the permalink), and title. You’ll probably want to pick whichever of those makes the most sense to you personally and use the same piece of data for all the pages, just so it’s easier to identify which pages it applies to if you come back to it later.

Do note that whatever code you write, you’ll want to write it inside a child theme. That’s because when you update your theme, it will overwrite any customizations you’ve done.