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!
Source: TPCi Rules Team (2020-08-13)

Relevant cards

Note: All applicable cards may not be listed below.
SWSH3_117
SWSH3_117
SWSH3_192
SWSH3_192
WOTCB1_72