I never concluded that Nagini was the same snake as the one in the first book. I recognized Nagini as a retic in the last movies, but at our house she is a spider ball python female.
I take most of those internet quotes from her with a grain of salt.
Also lets remember that Nagini is obviously a snake of magical properties, like Hedwig and Crookshanks (even though he is half-Kneazle). Because Nagini was capable of being a horcrux, then she may have had other magical properties bestowed on her. Maybe the venom is from He-Who-Must-Not-Be-Named's horcrux (soul fragment). I could see how they could be venomous.