PC_Ip - ip address of the PC
FW_IP(out) - any ip address within the subnet of the out interface of the FW(out)
if a traffic is initiated from the internet user and directed to the FW_IP(out), what will be the source IP address as seen by the PC?
The source IP address will be the original IP address of the PC on the Internet. You can NAT this to something else as it hits your firewall but in the example you give your are only NATting the destination address as packets arrive from the Internet user.