If I have Eternatus VMAX with "Eternal Zone" Ability in play and my opponent uses Celebi's "Time Spiral" attack 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!

Relevant cards

SM11_4

SWSH3_117

SWSH3_192