ED/Contagion Mechanic & DPS

Okay, so Contagion's unique mechanic is to spread Essence Drain on death. I'm liking this mechanic as it reminds me of the old Sacred 2 Dryad's skill 'Viperish Disease'. And the chain reactions Contagion/ED create are just so satisfying!
I got a couple of more in depth mechanic questions though:

1. Is the Contagion's on-death ED spread area of effect affected by increases to aoe or only the initial cast aoe?

2. Is the ED DoT duration reset on spread or is it one total duration based on the modified skill description?

3. Related to 2, do Contagion and/or Essence Drain benefit from increased skill effect duration and would stacking increased duration be advisable for either/both?

4. Is the unmodified ED DoT being spread by contagion or do debuffs on the first target that increase the DoT cause a higher damage ED DoT spread?
E.g. Skill ED DoT 1000/s with the first target being debuffed and cursed (wither stacks, despair) increasing the actual DoT on the first target to 2000/s. Will the DoT spread be 1000/s or 2000/s?

Generally the wiki is a good place to start :) Use this wiki as it is the one currently being updated: https://www.poewiki.net/wiki/Path_of_Exile_Wiki

To answer your questions:

1. Contagion's AoE is affected by increases to Area of Effect, on all procs. This means that Essence Drain spreads more easily with increases to Area of Effect.

2. The wiki states that 'When an enemy affected by both Contagion and Essence Drain dies, Contagion will proc and spread the remaining duration of both effects to nearby enemies.' (https://www.poewiki.net/wiki/Essence_Drain)

3. Increases to Skill Effect Duration would make your Essence Drain last longer, but based on personal experience you don't really need much, though some is nice for bosses etc. Generally your clear is limited by how far apart the monster packs are, rather than your Duration.

4. The unmodified damage is the one being spread.

Let me know if you have any more questions :)

Thanks PurpleChakra for taking the time to reply to all of my questions, appreciate it! :)

From what I've gathered from your replies, to max Contagion/ED map clear speed I should increase Contagion's AOE e.g. with increased aoe and intensify gems while increasing ED damage through buffs on my character rather than debuffs on mobs, e.g. arcane surge, void manipulation support, withering presence etc.

For bossing debuffs like temporal chains, despair, wither stacks etc and a longer duration are more important / QoL.

One thing I'm really torn about is which class/ascendancy to choose. I read guides on trickster and occultist and both have really good ascendancy nodes.

Occultist: Profane Bloom, Void Beacon and Withering Presence seem really good to debuff bosses to a greater degree and higher ED damage.

Trickster: Prolonged Pain and the movement speed buffs seem to be a good fit for a mapping focus.

Got any advise on class selection looking for an all in one mapping + bossing with Contagion/ED?