WordPress ate my HTML code

The control you want means learning how WordPress functions under the hood (that is, getting away from the admin area.) The kind of functionality you are looking for can be found in Page Templates. Duplicate the Page.php file of a theme, rename it to whatever you want and add

<?php
/*
Template Name: WhateverItsCalled
*/
?>

to the very top. You can write whatever HTML you want in it around the post template code (or delete the post template code, whatever.) Whatever page you intend to use that code on, when you edit it you’ll be able to choose the new page template from a dropdown menu on the right side of the screen.

Consider learning about WordPress filters (http://codex.wordpress.org/Plugin_API#Filters) and Plugin Development. Many plugins use some sort of bbcode shorttag in the text editor which will be replaced by content specified by your plugin.

For example:
This is text in my text editor. I am writing a page and I want to include a code snippet here so I will put a shorttag here which will be replaced. [myplugin snippet="1"]

check it out here http://codex.wordpress.org/Using_the_gallery_shortcode

hope this helps

Using page templates won’t affect anything that’s added by the WYSIWYG editor. It will let you structure, organize and style how that content is displayed and also lets you add content that can’t be touched through the text editor.

A page template looks something like this:

<?php
/*
Template Name: Generic Template
*/
?>
<?php get_header(); ?>
	<div id="posts-column">
	<div class="borderlayer">
		<?php if (have_posts()) : while (have_posts()) : the_post(); ?>
		<div class="post" id="post-<?php the_ID(); ?>">
			<h2><?php the_title(); ?></h2>
			<?php the_content('<p class="serif">Read the rest of this page &raquo;</p>'); ?>
			<?php wp_link_pages(array('before' => '<p><strong>Pages:</strong> ', 'after' => '</p>', 'next_or_number' => 'number')); ?>
		</div>
		<?php endwhile; endif; ?>
	<?php edit_post_link('Edit this entry.', '<p>', '</p>'); ?>
	</div>
	</div>
<?php get_sidebar(); ?>
<?php get_footer(); ?>

get_header, have_posts, the_title, etc are all functions of WordPress. From <?php if (have_posts()) : while (have_posts()) : the_post(); ?> to <?php endwhile; endif; ?> is called “The Loop”. Everything there is directly related to the content a user can edit. <?php the_title(); ?> is the title of the page and <?php the_content(); ?> is the content of the page (obviously). You can move them around within the loop to be displayed however you want them to be displayed. Outside of the loop you can add any other content you want, like the iframe.

I know it’s kind of confusing and I’m not sure I do it any justice, but once you try it out you’ll get what I mean and see that its exactly what you’re looking for. You don’t really have to know php to work with it.