Nope, set affixes aren’t affected on PVP. You can test this, take a Plagued (3) on PVP and see your HP being multiplied by 1.75 (75% increase).
Nova damage is OH based, so it inherents only the OH type of elemental crit.
Nope, Mythic affixes also don’t scale down on arena, meaning they have same DMG and Proc Rate at PVP. Even if it’s just 16%, the nova’s elemental crit is an effect, not an affix. Think about Stealth, the standard cap for dodge is 60%, but stealth allows you to have 100% dodge, because it’s an effect.
That means it isn’t updated yet. You must read patch notes for sure.