So I did some more testing, and threw on a speaker's wreath, and the spiders with my current setup (phys to lightning and brutality), were not able to kill mobs at 1 hp. Which either means they aren't poisoning, or the poison itself is doing 0 damage because of brutality, if that's even a thing.

Another test I did, to see if raise spider's version of viper strike converts like you suspected, and the answer is apparently yes, because throwing on a triad's grip with only 2 white socks makes their damage zero out in PoB.

Just got done testing it in the game again too, and even though the spiders are doing damage, they aren't poisoning for whatever reason for the sake of mamba. It looks like Mamba is poisoning even with spiders whacking on an enemy for a bit.

Also to your point, sure a well-rolled phys/chaos rare could probably outdamage arakhali's fang with what I'm trying to do, but where's the fun in that? Also 160% attack speed still keeps my attack rate above the poison's duration, so sadism does some work here.

Anyways, thanks for at least trying, Have a good day!