[2020] Question about the interaction between Elemental Overload and Explosive Arrow

There are two parts to Explosive Arrow - the arrow colliding with an enemy, and the explosion. The individual arrows colliding with the enemy hit and apply ailments in the same way as any other arrow and have no special mechanics. The stuff you've quoted is only in the context of the explosion, where one arrow exploding causes all the other arrows in the same enemy to calculate their hit and ailment damage for the explosion, then sums them together and applies them as a single hit.

All the damage for the explosion is calcualted at the time the explosion happens. Any relevant modifiers to that damage at that time will affect that calculation. Then the damage is combined into one big hit, and applied by the arrow that triggered the explosion (this matters if two different explosive arrow skills are in play for working out which on-hit effects apply).

At the time of the explosion, if you've dealt a crit in the past 8 seconds (and have EO), then each arrow will calculate it's explosion damage (both hit and ailment) with the 40% more from EO applying to it.

No. Each arrow calcualtes it's explosion hit & ailment damage separately. Only those which are crits individually will calculate their damage with that modifier, because that modifier only applies to damage calculation for crits.

When the individually calculated damages are combined, if any are a crit then the explosion as a whole will be considered a crit for on-crit effects, but this cannot affect the damage calculation in any way, because that has fundamentally already happened before that point.

Yes. The Damage-per-second of each ignite is calculated separately, then they are summed, then the exploding arrow applies one ignite with the total ignite damage per second, using it's own stats to work out the duration of the applied ignite.