Venom's damage rate (damage per frame, or second) and length are added to those applied by the javelins, but not the clouds, of Poison Javelin and Plague Javelin.
The usual rule of any previous poison being replaced by a later poison with an equal or greater damage rate applies; only one damage rate and length is applied at a time to a target.
In the absence of Venom, the damage rates of any items are added to those of the javelins, but not the clouds, of Poison Javelin or Plague Javelin and applied for the total length of items and skill divided by the number of items (so with a single item poison length is simply the sum of the item and skill lengths).
After switching weapons, any +% Poison Skill Damage will be applied to any clouds created after switching, and any -% Enemy Poison Resistance will be applied to any targets poisoned after switching.
K that makes sense. So the venom doesn't really do anything , as plague javelin is all about the clouds.
The switch target thing seems interesting, so hypothetically a DWeb would be a viable switch for a poison-zon? How about a wand with slvl 3 lower resist charges instead ( which seems more like a better option ).
Both are viable options: Death's Web's -(40-50)% Enemy Poison Resistance would also increase poison length by 40-50% (which might be redundant if poison damage is already high enough to kill without the added length), while a wand or dagger with level 3 Lower Resist charges would apply Poison Resist -41%, which could also remove many poison immunities (at least those that are PR 100% or PR 105%).
Hypothetically, you could buy or re-roll an Artisan's or Jeweler's Kris/Cinquedeas/Fanged Knife of Lower Resistance and socket it with three Rainbow Facet Jewels for +(9-15)% Poison Skill Damage and -(9-15)% EPR.