Magento-1.9 Price Backend Tierprice – Disable Tier Price in Backend Magento

No, you can't do that with a native Magento instance.
You would have to use shopping cart price rules for that, which are applied to the lowest of price, group price, tier price and special price.

I didn't find a simple method to load the index for a single product, the safest bet should be using a collection.

This helper method worked for me:

/**
 * Loads minimal price from index in single product entity
 *
 * @param Mage_Catalog_Model_Product $product
 * @return $this
 */
public function loadPriceIndex(Mage_Catalog_Model_Product $product)
{
    $productCollection = $product->getCollection();
    $productCollection->addAttributeToSelect([])
        ->addAttributeToFilter('entity_id', $product->getId())
        ->addPriceData();
    $product->addData($productCollection->getFirstItem()->getData());
    return $this;
}

Don't worry about database load, this results in a single query with only one join on the price index table like this:

SELECT `e`.*, price_index.price AS `indexed_price`, `price_index`.`price`, `price_index`.`final_price`,
    IF(`price_index`.`tier_price`,
        LEAST(`price_index`.`min_price`, `price_index`.`tier_price`), 
        `price_index`.`min_price`
    ) AS `minimal_price`,
    `price_index`.`min_price`, `price_index`.`max_price`, `price_index`.`tier_price`
FROM `catalog_product_entity` AS `e`
INNER JOIN `catalog_product_index_price` AS `price_index`
ON price_index.entity_id = e.entity_id
    AND price_index.website_id = '5'
    AND price_index.customer_group_id = 0
WHERE (`e`.`entity_id` = '2026')