Magento – Magento 2 How to send order email copy to custom email address with custom module when order placed

Oh that is a tough one. From what I can see you have two options.

1. Create your own version of the sendNewOrderEmail function so that you do the same stuff and generate the email but just send it to a different address.
2. Work with the $copyTo = $this->_getEmails(self::XML_PATH_UPDATE_EMAIL_COPY_TO);, I know you already have your event and I am not 100% sure what you need but you could change this function,

It might not be ideal but if you rewrite the Mage_Sales_Model_Order you could then check in the _getEmails if your conditions meet then you send a copy to where ever you need to, otherwise you continue with the normal sales order. Even as I write this I am not convinced if this is such a good idea, but I cannot see another "easy" way to hock into this without duplicating a lot of code when generating the email.

One final not is that the event you have selected checkout_onepage_controller_success_action is specific to the onepage checkout so if you create an order via the admin or multiple shipping checkout then your code would not be hit, sadly this has caught me out before :(

Here is a simple way of doing it.
Rewrite the method Mage_Sales_Model_Order::sendNewOrderEmail and make it look like this:

public function sendNewOrderEmail() {
    if (!$this->getForceSendEmail()) {
        return $this;
    }
    //do what it does by default
    parent::sendNewOrderEmail();
}

This will prevent the e-mail sending from the website because getForceSendEmail will always return false (unless you tell it not to).

Now, when running the cron to send e-mails you will have to do somthing like this:

$orders = ....; //retrieve orders somehow
$foreach ($orders as $order) {
    $order->setForceSendEmail(true); //tell the order to force the e-mail
    $order->sendNewOrderEmail();
}