I'm in the army, and my friend has agreed I download the game on his computer. I re-requested the CD key, placed it in the folder on his computer... but when I try to enter a server, it says CD Key authentication denied. Ideas?

If you are both going to play online, no, it won't work. The CD Key can only be used online by a single player. With the Bioware Authentication servers down, you can still get online with the same CD Key i think, but won't be able to play on the same server.

I don't mean to play simultaneously. I'm in the army, as I said - the other installation is at home. I'll only play through one computer at a time.
The odd thing is - I requested the CD key again, and I got 2 mails. One was 9 days ago, the other was 7, and they were different CD keys. Neither worked.

Ahh, ok, then it should work. You mentioned your friend, so i assumed you were going to play together with your friend online.

If you can get gog to re-send you the CD-Key thing, it should work, unless it's drastically different then the retail version's CD-Keys. Right now i have NWN installed on both my laptop and desktop, same cd keys, i play just fine, unless i try to log in with both computers at the same time.