Magento – Dashboard’s Lifetime Sales and Total Order Revenue Display Different Values

partial answer

1. Order in Backend does not fully affect rendering

• The totals are rendered by an "area".
• The grand total is in the area "footer". See template\checkout\cart\totals.phtml.
• Passing -1 to renderTotals() renders all areas.

So you have to remove $this->renderTotals('footer'); and change
$this->renderTotals(); to $this->renderTotals(-1);

Example (after copying totals.phtml to your own theme):

<table id="shopping-cart-totals-table">
    <col />
    <col width="1" />
    <tbody>
        <?php echo $this->renderTotals(-1); ?>
    </tbody>
</table>

After this change, the order configured in the back-end works as expected.

Based on your own answer, you can do it using Magento objects as Julien suggested:

$collection = Mage::getResourceModel('sales/order_item_collection')
    ->addFieldToFilter('product_id', $prodId);

$collection->getSelect()
    ->from('sales_flat_order_item', 
         array('sum' => 'sum(sales_flat_order_item.row_total_incl_tax)'
    );

$productRevenue= 0;

foreach($collection as $result) {
     $productRevenue += $result->getSum();
}

$productRevenue = $productRevenue / 2;