If I have Eternatus VMAX with the "Eternal Zone" Ability in play, can my opponent use Mawile GX's "Captivating Wink" Ability to play any non-Darkness Pokémon onto my bench?
No, if Eternal Zone is working, your opponent cannot force you to put any non-Darkness Pokémon into play.
Can I use both Victini's "Victory Star" Ability and Glimwood Tangle stadium to reflip coins from an attack twice?
No, you cannot. You can use either the ability or the stadium, but not both of them. As a general rule, you can't use more than one reflip effect at a time.
If I use Devolution Spray Z to devolve Eternatus VMAX with "Eternal Zone" Ability with enough damage already on the devolved Eternatus V to knock it out, is it KO'd before having to reduce my Bench down to 5 Pokemon, or if I discard it to reduce my bench is it not KO'd?
Well if the Eternatus VMAX is active it would get KO'd after reducing the Bench, but if the devolved Eternatus V is on the bench you could discard it as part of the bench reduction to avoid a KO.
If my only Eternatus VMAX with "Eternal Zone" Ability is Active and gets KO'd, do I reduce my bench size down to 5 Pokemon before or after promoting one to the Active Slot?
You discard down to 5 Benched Pokemon before promoting a new Active Pokemon in this scenario.
If I have Eternatus VMAX with "Eternal Zone" Ability in play and I use Devolution Spray Z to devolve one of my benched Darkness type Pokemon to a non-Darkness type Pokemon with enough damage already on the devolved Pokemon to knock it out, is it KO'd before having to reduce my Bench down to 5 Pokemon, or if I discard it to reduce my bench is it not KO'd?
You would discard your Bench down to 5 first, then check for a KO. That means you could discard the newly devolved Pokemon to avoid a KO, even if Eternatus VMAX is the one being devolved!
If I play Crobat V to the bench and use its "Dark Asset" Ability, can I use Super Scoop Up to bring it back to my hand then put it down to use "Dark Asset" again on the same turn?
Sorry, but the ability says that you may use ONLY ONE Dark Asset per turn, this applies regardless of whether any other Pokemon have used it.
Source:
Darkness Ablaze FAQ (2020-07-31), TPCi Rules Team (2020-08-13)
If I use Rhyperior V's "Drill Run" attack against a Darkrai with "Darkness Guard" Ability and only one Darkness Energy card attached, does Darkrai take 20 less damage before discarding the Darkness Energy or is the energy card discarded before the damage is reduced?
If an attack discards energy from the Defending Pokemon, the damage reduction happens first and then apply any attack effects.
Source:
Darkness Ablaze FAQ (2020-07-31), TPCi Rules Team (2020-08-13)