How to Add JavaScript Code to Head Tag for Success Page Only

The layout and blocks are kind of in a 'one direction' relation. I mean you can reference blocks in the layout but you cannot load layout files when creating an instance of a block. Actually you could but it's not the best idea to do so. Layouts should be called by the controllers.
When manually creating an instance of a block it has nothing to do with the layout files.
As for the second part, adding a js to the head, you have the right idea. It's done like you tried:

Mage::app()->getLayout()->getBlock('head')->addJs('js/path/here.js');

BUT... If the head block is already rendered then it has no effect. You have to make sure the js is added to the head after loadLayout() has been called in the controller action and before you call renderLayout() in the same action.

You can use this in your page.xml:

<action method="addItem"><type>skin_js</type><name>js/jQuery.min.js</name></action>

it should work, and load the js from your skin folder.

Much better idea than using xml to load jQuery is to load it in your header from a CDN go to your head.phtml and add this before <?php echo $this->getCssJsHtml() ?> :

<script src="//ajax.googleapis.com/ajax/libs/jquery/1.10.2/jquery.min.js"></script>
<script>!window.jQuery && document.write(unescape('%3Cscript src="<?php echo $this->getSkinUrl('js/jquery.min.js'); ?>"%3E%3C/script%3E'))</script>
<script>
    jQuery.noConflict();
</script>

I added jQuery.noConflict(); to fix the conflict with prototype. Also add jquery to your skin inside the JS folder, so if the CDN is not working the system will load it from your template