However, the idea of testing Requiescat on mobs that resist atonement is not a bad idea. The result could make a very clear distinction between the two.

I was out in abyssea kon as I was reading this thread, so I did a quick test.

T1 VNM, meanderer.

Atonement, 300% TP. Capped Hate.

375 DMG. Or a 50% reduction.

Requiescat: 900~1,500.
Had cruor buffs, But not DD atmas. (was playing with max possible DE+Holy II dmg ^^)
Just used my Chant set for ws.

For perspective, I used it a few times on the nearby gneiss leeches.
Gneiss leech + Req:1k~1.6k dmg.

Doesn't seem like req is dealing half dmg. So While It may be Non-Elemental DMG, it's clearly in a different class than Atonement.