Just walked a couple of potential new players through purchasing EE from GOG; they confirmed that a separate download of the 1.69 Diamond Edition was included, but they were advised that unique CD Keys were "depleted" and that the product team was going to work on the issue.
I think that, given how long this game has been out and how many people have purchased it from GoG, this might mean that there are literally no CD keys left for NWN 1.69, period. Remember, while CD Keys are long, they have to follow a pattern so they can be validated by the game. This pattern may not have even 32 bits of variation to it, meaning that while the pattern itself could be hashed into a 32-bit number, the variance of the pattern may be 24 bits (about 16 million variations) or less.
The only reasonable solution I can think of to this is the upcoming EE client support, which should include server-side support of EE CD Keys. This could be extended to allow the use of a Loader with the 1.69 client in order to use EE keys if desired, or in the event that doing so is easier than integrating full EE client support into HG (in which case supporting a loader that allows using EE CD Keys with the 1.69 client could be done ahead of EE client support).