WordPress 4.6.1 404 handling - php

Question

I have a challenge where you guys might help. I do have an REST interface from where get some JSON data to fill the necessary post / page data like headlines, copy and so on. On the other hand there's WordPress handling 404's on URIs that do not exists. So lets face the following scenario.
User requests the URI www.webpage.com/im-not-there/ and because that post / page doesn't exist it send you to the 404.php template. So far. so good, but this is where it becomes tricky.
Before I want WP to send the 404 I want to query the REST interface, asking it if the page exist (sure it's a bit more complicated as this ;) ). So possible answers from the REST interface could be TRUE or FALSE.
On TRUE I want to let WP go where it belongs, but on FALSE I want to "put" my data into the WP file. For that I do have the proper code and use the right hooks.
There might be the case that the post / page doesn't exist so the output (404) is correct.
Easier explanation:
User requests URI -> page / post exists -> requested template and WP stuff
User requests URI -> page / post doesn't exist -> ask REST interface and it returns
-> TRUE -> tell WP "don't use 404.php but use template-xy.php" and do your stuff as you're used to
-> FALSE -> standard WP behaviour.
Found out the following:
WP codex says cleary: "Loading a different template is not a good use of this action hook". The referenced hook is 'template_redirect'
Using template_include - see the codex example - causes further errors like the page is properly loaded but <title> is still from 404.php template
https://codex.wordpress.org/Plugin_API/Filter_Reference/template_include
Sure I can re-add all the neccessary filters like option_template and option_stylesheet and so on but is this really the way?
Codex says to the "wp" hook the following: "This hook is one effective place to perform any high-level filtering or validation, following queries, but before WordPress does any routing, processing, or handling. [...]"
From this point I'm not getting any further.
Further reading:
https://developer.wordpress.org/reference/classes/wp/handle_404/
Setup
Vagrant & PuPHPet
Nginx
MySQL 5.6
PHP 5.6
WordPress 4.6.1
No Plugins activated
Pretty URLs
Restrictions:
no core overrides
can't use server-side handling
using a filter on template_include is too late and causes further "errors" e.g. <title>
Sorry for not posting all codex links but Stackoverflow won't let me. :)
Thank you very much

Answer 1

Found the solution, I missed one but important step!
To set the HTTP header from 404 to 200 and also tell WP hat everything wen'T well.
So to use the codex example this is the solution on a 404
add_filter( 'template_include', 'portfolio_page_template', 99 );
function portfolio_page_template( $template ) {
global $wp_query;
if ( $wp_query->is_404 ) { // TRUE
$new_template = locate_template( array( 'no-404-template.php' ) );
if ( '' != $new_template ) {
if ($wp_query->is_404) {
$wp_query->is_404 = FALSE;
$wp_query->is_archive = TRUE;
}
header("HTTP/1.1 200 OK");
return $new_template ;
}
}
return $template;
}