[2021] Abyssal Echoes & Abyssal Decay Mechanics

For the first part (about damage effectiveness), yes to 1 and 2, and for 3 it's 150.5. All added damage, whether it's from items, a skill tree node, a passive tree node, attribute scaling, or anything else, is affected by the added damage scaling.

I'm mostly confused as this seems to be one instance where flat damage somehow impacts ailment scaling

Flat damage affects damage instance of the same ability type (melee, spell, bow, or throwing), e.g. flat melee damage affects instances of melee damage. Most ailments don't have an ability type tag, but Abyssal Decay does, it's spell damage, so your sources of flat spell damage apply to it.

The second part is if I allocate Potent Corruption (Abyssal Echoes hits but doesn't apply Abyssal Decay. 20 void spell base damage)... What is the damage effectiveness scaling at this point?

It flat spell damage applies to the hit damage at 100% effectiveness. This isn't stated on the node, but that's an oversight and should be changed.

Does Vorpal Reverberation no longer apply since the node says "Abyssal Decay deals additional void damage" so the Abyssal Echo wouldn't get the +10 flat damage?

It does apply, the wording is definitely unclear, but the actual effect of the node is to add +10 spell void damage to your cast of Abyssal Echoes and that affects all spell damage it deals, including the hit from Potent Corruption.