More that I thought of after my last post:
Nagini has to be older than Harry to be a horcrux. This is a fact (all the horcruxes but Harry had to be done before Voldie went after the Potters), since I doubt Voldie would use a baby snake less than a year old (not impressive enough).
The snake in book 1 was specifically said to have been BRED in the zoo and raised in captivity. Aka, Voldemort likely never got his mitts on it.
Also, I believe in the books that Nagini was mentioned as being venomous.
Boas aren't venomous.
*Headdesk* Rowling, just take back what you said right now before the reptile community embarrasses you.
Also, yosh for eidetic memory!