Neither of these scenarios doesn't work for situation where vendor admin provides a central shipping service to vendors and pays the costs of it. That's how we currently work and lot of high-end multivendor stores we know, that strive to provide a streamlined experience to the customer (a bit like ASOS).
Ok, I validated with my partners - actually we're not invoicing the shipping to the customer - vendors only invoice product costs. If some customer would insist on it, we (as a store) would invoice the shipping, but that's unlikely. I know that some other major stores with central shipping also operate this way.
We will think how to do it. The main issue is that the parent order is hidden in CS-Cart and customer, administrator and vendors can see only suborders. According to your logic, shipping cost will not be displayed anywhere