inserting content of 1 Post to in another with a template hierarchy

Yes, get_page_by_path() will return a post object or array on success, and not the post ID.

$faq_post = get_page_by_path( $post_slug, OBJECT, 'land' );  // object
$faq_post = get_page_by_path( $post_slug, ARRAY_A, 'land' ); // array

So there is no need to make the custom new WP_Query call, and your code below is good:

$faq_post = get_page_by_path( $post_slug, OBJECT, 'land' );
$content = $faq_post->post_content;

However, if I understand you properly, the $post_slug should be:

$post_slug = 'landing-faq-' . $post->post_name;

And check that $faq_post is not a null (which indicates a failure):

$content = $faq_post ? $faq_post->post_content : '';

And it might be faster to perform a custom SQL query to search for the post with that slug:

global $wpdb;

$row = $wpdb->get_row( $wpdb->prepare( "
    SELECT ID, post_content FROM {$wpdb->posts}
    WHERE post_name = %s AND post_type="land"
", $post_slug ) );

if ( $row ) {
    $content = $row->post_content;

    // If you need to apply all filters to the content.
    $content = apply_filters( 'the_content', $content );
    $content = str_replace( ']]>', ']]&gt;', $content );
} else {
    // Here you can grab the content of the default post.
}

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