I have bought all the NWN games a long time ago, including the infinite dungeons and pirates of the sword coast. The install CDs are no longer working so I repurchased it via GOG. I modified the nwncdkey.ini file to match the CD keys with which I purchased POTSC, installed it, and tried to run. But it doesn't authenticate.

1. Was I correct in assuming that
Key 1 = Neverwinter Nights
Key 2 = Neverwinter Nights: Shadows of Undrentide
Key 3 = Neverwinter Nights: Hordes of the Underdark

2. If so, how to I get it to work?

EDIT: The issue is that the DRM servers are down: http://social.bioware.com/forum/1/topic/199/index/7715716/1

EDIT 2: The servers were brought back online by EA a couple of months after I made this post, as of early 2014 they are still working.

Ok, I think I found the issue.
I found an original copy on my laptop and:
1. Yes I was correct in my assertion on which key is which game.
2. It doesn't work there either.

In fact, I looked it up, patch is no longer available via auto update and servers are just down, supposedly something to do with hacking.
If the servers ever come back up I will reattempt this and see if pirates of the sword coast works with the GOG version or not.

This was only a matter of time since they disabled the shop that allowed the purchase of the premium modules a few years ago. Now with all the NWN stuff gone (not only because of the hacking, they even restructured their site so you can't even find a link to the NWN patches anymore) i don't think the chances are very good to ever be able to auth the modules again. They should just release them all for free i think - as a last "thank you" to all the fans. But keeping the old fanbase together isn't on the priority list anymore since EA took over, it seems...

When you installed TotSC the first time it attached itself to the NWN CD Key you had at the time. In order to use it you must also use your old CD key. It's easy to do this by hand. Just open the ini file with notepad, change it, and save the changes (make sure it saves as ini and not txt).

I'm pretty sure you only need to worry about the NWN CD Key. I don't think it looks at SoU or HotU.

I am well aware of that and I did that already.
It doesn't work because the servers are down.

Understanding this is a long dead thread, I thought I'd throw this out there for interested parties who may be looking:

If you have the installers for this old Premium DLC, they work with the GOG version of Diamond. I've played PotSC and Infinite as recently as today.

A relatively quick search of BioWare and EA threads suggests that the old DRM auth servers for BioWare were rebuilt, and for this content they were set to just "authenticate anyone" (that's hearsay, not gospel).

So, if you have the old install packages, give it a try. Mine worked fine with GOG.

And if you don't have them, you can get them here.
http://social.bioware.com/forum/1/topic/199/index/4746466
The links still work.

Actually what happened is that a few months after I made this thread EA brought the DRM servers back online. And now they authenticate for everyone even if you didn't actually buy it

You CAN install the DRMed version of POTSC with the gog version and it will work so long as you have an internet connection for it to authenticate and so long as the DRM servers remain online.

Also, at the moment EA actually stopped keeping records of who owns what and the server now just automatically authenticates for everyone.

Biowares website hosts the old premium as linked by olnorton. It was originally trialware. Where you download the module, you run it, if it authenticates you get the full premium module, if it fails to authenticate you get a playable demo. However at the moment the only way to get the playable demo is to block it in your firewall or disconnect from the internet since as long as you got an unimpeded internet connection it will always authenticate for everyone