Manual post order arrangement - php

Question

My goal is to allow exact, sticky, front-page only, positioning of posts based on user back-end input. All other WP behaviour should remain the default.
The front page displays 9 posts
User can select the position of the post on the front page (Off, 1-9)
Only one post can have an assigned position, the posts with the
latest post date is selected, others with the same positioned are
ignored and remain part of the regular orderby flow
In order to re-arrange my posts I run a second WP_Query to collect all posts that have a position assigned to them through a custom meta field. The meta field shows the option 1-9 on the admin post page but corresponds to the index numbers of an array, just like the WP_Query object posts array.
function get_positioned_posts() {
$positioned_posts = get_posts( array (
'meta_query' => array(
array(
'key' => 'post_position',
'value' => array('0','1','2','3','4','5','6','7','8'),
'compare' => 'IN',
),
),
));
return $positioned_posts;
}
Subsequently I get the MAIN posts array, loop over it and insert a positioned post when the index number matches the positioned_post value.
function mns_post_position( $posts) {
if ( is_home() && is_main_query() && ! is_admin() && ! is_search() && ! is_archive()) {
$positioned_posts = get_positioned_posts();
$positioned_post_ids = wp_list_pluck( $positioned_posts, 'ID' );
$num_posts = count( $positioned_posts );
// Find the positioned posts
for ($i = 0; $i < $num_posts; $i++) {
if ( in_array( $positioned_posts[$i]->ID, $positioned_post_ids ) ) {
$positioned_post = $positioned_posts[$i];
// Remove post from current position
array_splice( $posts, 4, 1 );
// Get post position and put it in the posts array
$position = intval(get_post_meta( $positioned_post->ID, 'post_position', true ));
array_splice($posts, $position, 0, array($positioned_post));
}
}
}
return $posts;
}
add_action('the_posts', 'mns_post_position', 1 );
This works great, but it only works for the homepage and not the paged pages. The "the_posts" action only gets the current X (9) posts and not the entire WP Query posts array. This causes unwanted behaviour:
There can be more than 9 posts on the frontpage if a positioned posts
comes from page > 2.
You can see a positioned posts twice, once on the frontpage and later
one a paged page.
Possible solutions
I need to find a way to build the query correctly on the first try.
I don't see how I can achieve this with pre_get_posts filter.
Using "menu_order" like some plugins is not an option. Ordering by date is required.
Find a hook that returns the ENTIRE WP_Query Posts array, which I
just rearrange with array_splice.