I own and am in possession of all my retail disks and manuals for Neverwinter Nights, plus expansions. I have my keys for base NWN and HotU. I do not have my SoU key, I was one of the unlucky ones who had their key printed on the little red paper slip which wasn't in my retail box.

That said, here is my question. I bought Infinite Dungeons way back when. Can I use my retail NWN key to still authenticate my game and play ID, which is bound to that key, while using the SoU key provided me by purchasing the game here in order to install?

I can't see any reason why that wouldn't work.