Set/affixes with the same effect

+250% Reflect Damage does Arcane damage, no matter what the MH Element is. It doesn’t do Elemental Crits at all! it can do Critical Hits, Crushing Blow, and I think it can also cause Bleed if you have Bleed Chance on your Build somewhere. +400% Explosive also works with it.

Weaken works with it, and so does Effective Mythic.

Reflect Damage doesn’t have a Cap. anything that increases the global damage will increase the damage of RD.

Elemental Damage is calculated for the Weapon before Reflect Damage.

yes for the under 25% health question.

all 3 Classes have at least 1 Talent or Skill that affects the damage of RD.
Electrified Set increases RD by +50% per Rank and also turns each RD into a 25 yard radius AoE centered on your Character.
Redirect Mythic allows you to RD when you Dodge or Block an attack, and increases RD +100% when you Dodge or Block.
Mirrored Mythic increases the damage of RD by your Block %.

hope this helps.