Wordpress stripping attributes even when explicitly allowed - wordpress

Question

I've looked through every guide on allowing HTML tags and attributes to Wordpress posts, but it's still stripping the allow attribute from iframes. When echoing the allowedposttags global, it looks like iframe[allow] is set to true. TinyMCE's settings show that extended_valid_elements is set to allow all *[*]. The POST data to the Wordpress update endpoint is not stripped. I'd really appreciate any help in identifying the problem. Here's the plugin I put together based on a few guides:
<?php
/**
* Plugin Name:Allow Tags
* Description: Allows one to add "invalid" and custom HTML elements to the Wordpress editor.
* Version: 0.1
* Author: Sam Hill
* with help from http://www.engfers.com/2008/10/16/how-to-allow-stripped-element-attributes-in-wordpress-tinymce-editor/ and many other sources
*/
class AllowTags {
public static $instance = null;
public static function get_instance() {
if ( null == self::$instance ) {
self::$instance = new self;
}
return self::$instance;
}
private function __construct() {
add_filter('wp_kses_allowed_html', 'custom_wpkses_post_tags', 10, 2 );
add_filter('tiny_mce_before_init', 'tinymce_allow_iframe');
add_action('init', 'allow_iframe_attributes', 10, 2);
}
}
AllowTags::get_instance();
function tinymce_allow_iframe($init) {
$options = '*[*]';
$init['valid_elements'] = $options;
$init['extended_valid_elements'] = $options;
$init['verify_html'] = false;
return $init;
}
function custom_wpkses_post_tags($allowed, $context){
if (is_array($context)) {
return $allowed;
}
if ($context === 'post') {
$allowed['iframe'] = array(
'src' => true,
'height' => true,
'width' => true,
'frameborder' => true,
'allowfullscreen' => true,
'allow' => true
);
}
return $allowed;
}
function allow_iframe_attributes( $string ) {
global $allowedposttags;
$allowedposttags['iframe'] = array(
'allow' => true,
'align' => true,
'frameborder' => true,
'height' => true,
'width' => true,
'sandbox' => true,
'seamless' => true,
'scrolling' => true,
'srcdoc' => true,
'src' => true,
'class' => true,
'id' => true,
'style' => true,
'border' => true,
);
return $string;
}
}