Easy way to find out what template files are being used .... look at the main_page variable in the URL.
The page you want is 'shopping_cart'.
1a & 2) I'm not sure what you mean by 'main page'. In your admin under Configuration --> Stock are the switches to turn the New, Featured, Special, and Upcoming products sections display on and off and move them around. If you mean that you only want featured items to appear on the index page, set 'Show Featured Products on empty Shopping Cart Page' to 0.
1b) To limit the number of Featured products displayed, go to Configuration --> Maximum Values and set 'Maximum Display of Featured Products - Main Page' to 1. It will automatically be randomized.
3) In includes/templates/YOUR_TEMPLATE/templates/tpl_shopping_cart_default.php, add the HTML that you want after this line (around line 182):
Code:
<h2 id="cartEmptyText"><?php echo TEXT_CART_EMPTY; ?></h2>
BrandTim