First, though each copy of NWN that GoG comes with the same CD key, GoG gives out unique CD keys to purchasers who request them. So, all of your players with a legit copy of the game should have their own CD keys. In other words, don't worry about linking CD keys to player names because of the generic CD key, since people playing online should not be using that one.
As far as I know, lots of people still have IP addresses assigned unpredictably when they reboot their routers. Not that they get a new IP address each time necessarily, but their is no guarantee that they get the same IP. Even aside from the IP assignments of the ISPs for players' home internet connections, players will have a different IP when they want to play from a friend's house, while traveling and staying at a hotel, or even just when checking in from a WiFi hotspot somewhere. I think server security that assumes players will have the same IP each time they log in is going to result in lots of headaches and lots of frustrated players.
I am not a server admin, so others will have other advice as well. But, I would suggest reading FunkySwerve's post in the "Securing Your Server Without Master Server Authentication" thread in this forum. It provides a nice way to link player names to CD keys (even accommodating players with more than one key, as many of us do) to provide at least a baseline of security.