Unable to send user-entered data to PayPal along with credit card info
From your above code i have seen that dpm($form) showing 2 fields. when you submit your data in form please ensure that your order is created or not. Becuase in your code you are getting customer order details and pass it to the array() (which is empty). Thats why it not going to Paypal website. Please check your custom code.

hook_commerce_paypal_api_request_alter(&amp;$nvp, $order, $payment_method):  What this function will do, it will alter the array before pass this data to paypal website. So you must ensure that array data you are passing to paypal is not empty or you can check the condition for it.