Not so long ago, GOG was giving customers a free DLC of NWN ver 1.69. I am not on a Mac, but it seems to be working just fine. When I buy games from GOG (Good Old Games) they remove the need for CD keys. I just run the installer and everything works. I have never had any issues with the company to that effect.
I don't know about modding the existing models but the built in game editor seems to work well. The only thing I looked up on the internet was my portrait which you can customize with a paint program such as GIMP. You have to add it using commands from the console.
Multiplayer is limited to LAN, as far as I know, there is no official network to hook up with other players. You will be prompted for name and password, it takes less than a minute for the game to give up and ask you about a LAN game. This is where you will need to hook up via IP address 192.168.1.## for local stuff and if you plan a long distance connection you are going to need your friend's IP and they will need yours.
I also get a "Game Spy" screen when selecting the Join Internet connect, I know even less about "Game Spy" but that is up to you to decide to use or not. But the direct connection should work, I just never tested it. My hubby is more computer aware than I, and told me you could open a can of worms with a shared IP. So you need to trust the individuals you play with on the other end.
I hope this answers some or most of your questions. NWN is a great game, I was playing NW D&D but now I have to access it only through my HP Laptop on Windows 10, it seems they upgraded last year and made the game DX 10 and 11 only, but still runs in a 32 bit environment. My laptop is having issues now that the tech support can't answer. :(