use a custom page to display search results

I noticed that your goal is to have an editable content on the search result. You can use another method to do that, by calling the content of the page from your search.php file.

You can start by creating a page with the slug search-page as usual. You can also add post meta / custom fields as you need.

And now you can start editing the search.php. Inside the search.php get the page content by using

$page = get_posts(
    array(
        'name'      => 'search-page',
        'post_type' => 'page'
    )
);

Then use the page data inside the $page object in your template. If you need the page meta (custom fields), just use the get_post_meta() functions.