Hi,
1. You can potentially edit the view "checkout / step" and at the top of the code, add something like that:
<?php
if($this->step == '3'){
$this->display_checkout_bar = false;
}
?>
Please replace "3" by the step number of the thank you page.
2. It depend on the payment method used, and its setting, for example the order status once payment done.
Some methods will use the "checkout / end" page, some others don't. If the payment method got a "xx_end.php" file so this one will be used, else the "checkout / after_end" can be called. This is based too on the return url set in the settings of the payment method.