Magento 1.8 – Event of Clicking Checkout Button

You can accomplished this by overriding Mage_Sales_Model_Quote_Item via modules config.xml:

<global>
    <models>
        <sales>
            <rewrite>                    
            <quote_item>Namespace_Module_Model_Quote_Item</quote_item>
            </rewrite>
        </sales>
    </models>
</global>

And then overriding the calcRowTotal()

class Namespace_Module_Model_Quote_Item extends Mage_Sales_Model_Quote_Item
{
    protected $customRowTotalPrice = null;

    public function setCustomRowTotalPrice($price)
    {
        $this->customRowTotalPrice = $price;
    }

    public function calcRowTotal()
    {
        if ($this->customRowTotalPrice !== null) {
            $this->setRowTotal($this->getStore()->roundPrice($this->customRowTotalPrice));
            $this->setBaseRowTotal($this->getStore()->roundPrice($this->customRowTotalPrice));
            return $this;
        }

        $qty = $this->getTotalQty();
        $total = $this->getStore()->roundPrice($this->getCalculationPriceOriginal()) * $qty;
        $baseTotal = $this->getStore()->roundPrice($this->getBaseCalculationPriceOriginal()) * $qty;

        $this->setRowTotal($this->getStore()->roundPrice($total));
        $this->setBaseRowTotal($this->getStore()->roundPrice($baseTotal));
        return $this;
    }

}

There are multiple approaches how you can do this without editing magento code (webserver level, for example). These approaches might be more efficient.

But if you want to do it with magento, your $resultRedirect should be returned to App\Http. Your observer does not return $resultRedirect, so it is not processed by \Magento\Framework\App\Http::launch and redirect does not happen. Since you want your redirect logic to happen as early as possible, but only in storefront (is that right?) instead of observer you should put an around-plugin on \Magento\Framework\App\FrontControllerInterface::dispatch because it's the first method that gets called in \Magento\Framework\App\Http::launch after area is defined and returns ResultInterface. From that plugin you will be able to return your $resultRedirect object:

namespace My\Module\App\FrontController;

class Plugin
{
    // Constructor and dependencies here

    public function aroundDispatch(
        FrontControllerInterface $subject,
        callable $proceed,
        RequestInterface $request
    ) {
            $baseUrl = $this->storeManager->getStore()->getBaseUrl();
            $gi = geoip_open(getcwd()."/app/code/Amit/Geoip/Observer/lib/GeoIP.dat",GEOIP_STANDARD);
            $ip = $_SERVER['REMOTE_ADDR'];

            $country_code = geoip_country_code_by_addr($gi, $ip);

            if((strtoupper($country_code) == "IN")){
                $resultRedirect = $this->resultFactory->create(ResultFactory::TYPE_REDIRECT);
                $resultRedirect->setUrl($baseUrl . '/in/' . $_SERVER['REQUEST_URI']);
                return $resultRedirect;
            } else {
                return $proceed($request);
            }
    }
}

As for doing it only for first customer, cookie presence check looks like a viable option.