Assuming one could be found, would the preorder code still be valid? Vaguely pondering the idea of getting a CE but wouldn't mind access to the fancy crystals either if I could find a working code for that too.

Oh sure, i just did that a couple of months ago, bought a CE edition on eBay, used the code, and voilà, CE items and everything

You want to make sure that the box (or at least the game disc case) is sealed though, so that the code has never been used !!