Ajax the create and edit post form into lightbox and get results

Have you considered using the ACF plugin?

With that great plugin (no connection, i just can’t imagine WP without it) you have the function acf_form to render any content form on the frontend, all with ajax updates and saving (works also in a popup via IFrame)

Even though the sidebar elements such as tags and categories are missing, you could easily replace them with custom ACF fields.

The following example is taken from their page and worked perfectly for me:

<?php acf_form_head(); ?>
<?php get_header(); ?>

    <div id="primary">
        <div id="content" role="main">

            <?php /* The loop */ ?>
            <?php while ( have_posts() ) : the_post(); ?>

                <h1><?php the_title(); ?></h1>

                <?php the_content(); ?>

                <?php acf_form(); ?>

            <?php endwhile; ?>

        </div><!-- #content -->
    </div><!-- #primary -->

<?php get_footer(); ?>

Make sure to read the documentation to see how to activate editing of the standard title and text fields and not only your custom fields, this example will just let you edit the ACF fields