Magento – Can anyone suggest tutorial for creating own Magento theme

I think you are missing some DOM elements and/or ids. For example in your checkout there is not element with ID: 'billing-buttons-container' or 'shipping-buttons-container'. I think these are the elements that are null when trying to set some properties on them.
You might be missing other elements also. I recommend to compare your checkout templates with the default templates.

The following code worked perfectly for me to create Grouped product with it's associate products. Hope this will help someone to create grouped products programmatically.

public function createGroupedProduct()
    {
        $sku = 'AL108';
        $title = 'my test product';
        $description = 'this is a description about the product...';

        $product = new Mage_Catalog_Model_Product();

        $product->setSku($sku.'-grouped');
        $product->setAttributeSetId(63); // put your attribute set id here.
        $product->setTypeId('grouped');
        $product->setName($title);
        $product->setCategoryIds(array(35)); // put your category ids here
        $product->setWebsiteIDs(array(1,2,3));// put your website ids here
        $product->setDescription($description);
        $product->setShortDescription($description);
        $product->setPrice(1000);
        $product->setWeight(200);
        $product->setVisibility(Mage_Catalog_Model_Product_Visibility::VISIBILITY_BOTH);
        $product->setStatus(1);
        $product->setTaxClassId(0); 
        $product->setStockData(array(
                                    'is_in_stock'             => 1,
                                    'manage_stock'            => 0,
                                    'use_config_manage_stock' => 1
                                    ));

        try {
// Save the grouped product.
            $product->save();       
            $group_product_id = $product->getId();

// You need to create an array which contains the associate product ids.
            $simpleProductId[0] = 1483;
            $simpleProductId[1] = 1484;
            $simpleProductId[2] = 1485;
                $simpleProductId[3] = 1486;
            $simpleProductId[4] = 1487;

            $products_links = Mage::getModel('catalog/product_link_api');

// Get grouped product id.
            $group_product_id = $product->getId();

// Map each associate product with the grouped product.
            foreach($simpleProductId as $id){
                $products_links->assign ("grouped",$group_product_id,$id);
            }

        } catch (Exception $ex) {
            echo $ex->getMessage();
        }

    }