Hi,
Don't worry about the "show_default.php" content, we already have it
Just searching "description" in the file and you would see the block:
<div id="hikashop_product_description_main" class="hikashop_product_description_main">
<?php
echo JHTML::_('content.prepare',preg_replace('#<hr *id="system-readmore" */>#i','',$this->element->product_description));
?>
</div>
Which display the product description.
You can move this block into another HTML block.
The current HikaShop CSS display the "hikashop_product_bottom_part" below the "hikashop_product_left_part / hikashop_product_right_part" parts.
So, you can move the block at the bottom of the "hikashop_product_right_part" or you can modify the CSS in order to display the bottom part like the right part (using "float" rules, removing "clear" rules).
Regards,