You can , in fact, give the promo to a different account. It's not tied to the account that bought the phone.
Have you tried it?
I used the link in the email and it went to a page that added my credit without entering the code. Could be that the code was embedded into the URL.
I got my $50 Google Play Credit email yesterday,a day after getting the 5X
Now what about the $69 Nexus Protect?
Do you have a receipt for Nexus Protect? If so, you have legit evidence in case anything happens. IF you do not, this program is via a different company and not actually Google or Nexus at all. I assume they have you listed somewhere to eventually contact with your account info. Could be that they were overwhelmed.