[3.1]Shaper Down on budget 4-link –That Poet's Pen(“instant leech”_Glacial Cascade & Firestorm 2in1)

So I played a dual curse ele/sla version of this back in Bestiary, and really enjoyed it, but sometimes felt the single target damage was lacking a bit. I also thought the new Herald of Agony looked pretty interesting. And Firestorm with Volkuur's is really good at poisoning. Seemed like a combination worth trying, this is what I've got so far.


PoB:
https://pastebin.com/AvGt0WGc

Gear


So, yeah, I'm liking it so far, but wanted a few opinions.

1) Is Volkuur's actually needed, or would the 60% combined poison from assassin and HoA with some added phys/chaos to spells be enough?

2) On PoB replacing cold penetration with vile toxins increases damage at about 4-5 stacks on a normal enemy, 6-7 stacks on Shaper. Worth doing?

3) Does anyone have a definitive answer on what type of damage the crawler does? I've heard full phys, I've heard 60% phys 40% chaos. Again, I'm wondering if it might be worth replacing vicious projectiles with vile toxins?