Hi,
1.Yes.
2. The preload area is not supposed to be modified unless you're a developer.
And if you're a developer, you can easily know what variables are available in it. Just look at the code.
For example, you have a $data variable there which contains all the order data.
If you want to know what it contains and how to use it, just add such line:
<?php var_dump($data); ?>
then have your website send the email and it will display all the content of the variable in the email and that way you'll know how to call the data you need.
In there, you can call any function of HikaShop. So if you know it, you can call it. Like in any views of HikaShop.
And HikaShop doesn't have any global variables. $data isn't a global variable. It's a variable set by HikaShop before including that file. It's the same principle as $module when you are in a module's main file. That variable is set by Joomla just before calling the module file so that the module can access its own data.
$data contains all the necessary data to display the email. There is no other variable.