Rewrite rule and display of post

Question

I'm trying to create a permalink for single posts that are in a particular category called 'Test'. So I've set the stub with a filter which in turn sets the permalink when the post is saved. At the moment I have it hardcoded to 'test/post1'. Stub and permalink set just fine. I then have a rewrite rule which writes 'test/post1/' to 'index.php?pagename=test/post1' (again hardcoded just to test).

It looks like it's sort of working but wordpress only shows me the index with no posts (404) - so I'm seeing the all posts page rather than the single post page. Anyone know what might be wrong?

add_filter( 'wp_unique_post_slug', 'filter_wp_unique_post_slug', 10, 4 ); 
add_filter('rewrite_rules_array','wp_insertMyRewriteRules');
add_filter('init','flushRules'); 

function filter_wp_unique_post_slug( $slug, $post )
{
// Get the categories for the post
$category = get_the_category($post->ID); 
if (  !empty($category) && $category[0]->cat_name == "Test" )
{
    $slug = 'test/post1';
}
return $slug; 
};

// Adding a new rule
function wp_insertMyRewriteRules($rules)
{
$newrules = array();
$newrules['(.*)/(.*)/$'] = 'index.php?pagename=test/post1';
return $newrules + $rules;
}

// Remember to flush_rules() when adding rules
function flushRules(){
global $wp_rewrite;
$wp_rewrite->flush_rules();
}

Edit 1

@Milo, I had tried the permalink filter but when it didn't work and I noticed the slug was different I thought there may have been an issue there. I used this (below) previously. The permalink looks like this 'test/post1' and the slug like this 'post1'. Now I see the all posts page with the correct post preview on its own - it's still not the single post page of that post.

add_filter( 'post_link', 'custom_permalink', 10, 3 ); 
add_filter('rewrite_rules_array','wp_insertMyRewriteRules');
add_filter('init','flushRules'); 

function custom_permalink( $permalink, $post, $leavename ) {
  $category = get_the_category($post->ID); 
  if (  !empty($category) && $category[0]->cat_name == "Test" )
  {
      $permalink = trailingslashit( home_url('test/' . $post->post_name ) );
  }
  return $permalink;
}

function flushRules(){
  global $wp_rewrite;
  $wp_rewrite->flush_rules();
}

function wp_insertMyRewriteRules($rules)
{
  $newrules = array();
  $newrules['(.*)/(.*)/$'] = 'index.php?name=post1';
  return $newrules + $rules;
}

Answer 1

I don't understand why but the regex seemed to be the last issue. It was catching it but even with the post name hardcoded in it didn't work correctly. This is the final working version.

add_filter( 'post_link', 'custom_permalink', 10, 3 ); 
add_filter('rewrite_rules_array','wp_insertMyRewriteRules');
add_filter('init','flushRules'); 

function custom_permalink( $permalink, $post, $leavename ) {
  $category = get_the_category($post->ID); 
  if (  !empty($category) && $category[0]->cat_name == "Test" )
  {
      $permalink = trailingslashit( home_url('test/' . $post->post_name ) );
  }
  return $permalink;
}

function flushRules(){
  global $wp_rewrite;
  $wp_rewrite->flush_rules();
}

function wp_insertMyRewriteRules($rules)
{
  $newrules = array();
  $newrules['^test/(.*)$'] = 'index.php?name=$matches[1]';
  return $newrules + $rules;
}