1) use a comma separated list of the columns to include in the ORDER BY Clause:
"ORDER BY ProductPrice, ProductEuroPrice DESC"
2) Use an if statement around the add to cart button to show an Out of stock error message:
<?php if($row_recordsetName['ProductLive'] != 0) { ?>
[add to cart button code]
<?php } else { ?>
This product is out of stock
<?php } ?>