Isnt the "return within 5 days" just the precaution so you dont get a break in service (access to GW2) at launch? They are giving you a 5 day buffer to pickup your final game (and key), if you miss the buffer then you lose access to the game post launch until you actually go pickup the game and resume your access.
This will be my first MMO launch, how do other MMOs handle prepurchase/early starts with physical releases? Surely they must have something in place to prevent someone from prepurchasing, getting the beta/early access code, and then cancelling the order right?
No idea. Some do, some don't. I pre-purchased Tera for beta access in early February, decided I didn't like it and just this weekend requested and got a refund for the game. I don't really see ANet locking people in, even if they did pre-purchase, received and used their codes.