retrieve order_id onBeforeOrderCreate

The event onBeforeOrderCreate is called, as the name tells you, before the order is created. So you don't have the order_id at that time since the order_id is generated automatically by MySQL when a new entry in the order table is inserted.
If you really need it at that point, the only way I can see (which is not perfect), would be to do a query and load the max order_id of the order table and increment it so that you get the next order_id which will be used and there is a very high probability that there won't be any order created between that time and the creation of the order.
But maybe you should do it differently: add a custom order field and use the unique_id() PHP function to generate an unique id that you can put in the custom field, use it in your code, and then you will be able to identify the order with that unique id.

That's also a possibility indeed.

You have all the information you want including the cart_product_id but cart_product_id is for each products of the cart. So it's in a products array and not directly in that object. For example, for the first product of the cart:
$order->cart->products[0]->cart_product_id;

As a general advice, you should include the library dbug on your website: dbug.ospinto.com/
and then use that code:

new dbug($order);
exit;

That will display you nicely the whole object and you will be able to easily see where the information you're looking for is stored.

I just checked it on our end and the cart_product_id of the products is not given to the event onAfterOrderCreate.