Trash a post, send associated comments into the trash bin (change status)

There’s an action called trashed_post_comments that runs right after the comments are set to post-trashed status. You could hook into that:

add_action( 'trashed_post_comments', 'wpse134528_really_trash_comments' );
function wpse134528_really_trash_comments( $post_id ) {
    $args = array( 
        'post_id' => $post_id,
        'status' => 'post-trashed',
    );
    $comments = get_comments( $args );
    foreach( $comments as $comment ) {
        wp_trash_comment( $comment->comment_id );
    }
}

References

• trashed_post_comments action hook
• get_comments()
• wp_trash_comment()

Hopefully this helps, though I’m not sure I understand why comments with their status set to post-trashed would be messing up your theme…