1. No problem

2. Yes. The server should store all ports of the clients in a table. If a new client connects, you could send the whole table to all participants for example.

3. Yes, that's basicly what NAT/PAT is doing. This technic allows you to access the internet through more than one pc at the same time, although you only have one internet ip address. You can find more informations about that in wikipedia.

4. Every packet contains the source and destination port. You are right, this is currently not possible because the sock_recv() function doesn't return the destination port. This will be changed!