So, the right column is displayed by the quantity file of the view product. That same view file is also used to display the stock & button on the product page.
The two <br/> which make them display on two lines are in that code of the file:
echo '<span class="hikashop_product_stock_count">'.JText::sprintf('X_ITEMS_IN_STOCK',$this->row->product_quantity).'</span><br/>';
$config =& hikashop_config();
if($config->get('button_style','normal')=='css'){
echo '<br />';
}
If you remove the <br /> tags there, then you can just add some CSS:
.hikashop_product_add_to_cart_row{white-space:nowrap;}
and everything will be on one line on your listing.
However, the stock & button will also be on one line on the product page if you just remove the br tags. So instead you should have a check like that...:
if(JRequest::getVar('task')=='listing'){
//no br
}else{
//display br
}