This is correct.
So, if I SS into a mob, that would apply 1 poison from SS’s weapon damage component, 1 from NJE & the frostburn/vitality decay from nightfall (though any poison from the weapon damage component would be ignored initially as the poison from SS itself would take precedence).
Also correct.
Then if I keep on hitting the mob, I’d get poison stacks from the default attack (which would only do damage after the initial poison from SS has run out), plus the poison’s from NHH as delivered/applied by any WPS that proc & any poisons from the weapon damage components of the WPS (again, they get applied after any higher damage weapon damage-based poisons have run out, in descending order of damage per sec).
More or less right here.
How are the weapon damage poisons deal with, do they all get applied eventually, one at a time or is there a time limit which could make very old poisons get ignored/dropped?
Can you elaborate on what you mean here?
From post #45/46, all gear/devotion/passive skills (not including NHH, as that would only apply to the WPS) that give flat poison damage get added together (so, the dots from the Ring of the Black Matriarch, Vilioth’s Bile, Merciless Repertoire, various devotion nodes), buffed by any +% poison/all damage & then applied any time I poke anything with a pointy stick (further buffed/nerfed by any +% weapon damage on the WPS that happen to proc)
Correct.
On the second character tab, where it says x-y poison damage & the row below gives a +z% poison damage, is the x-y damage the figure after % damage bonuses have been applied, or before? And is that x-y poison damage per sec (presumably over 5 secs, plus any duration bonuses)?
X-Y damage is after % bonuses are applied. Damage is per second over 5 seconds excluding % duration bonuses.