1) In addition to IP address you also have to change the destination MAC address if proxy is located in the same network segment as you are.
2) You have to memorize the connection {source IP, source port, original remote IP, original remote port, new remote IP, new remote port}. When receiving a response on the redirected packet you have to find the connection in your connections table by destination IP/port which in your table should match source IP/port and substitute source IP/port in the packet.
