Yeah Shadowfell stacks with Darkhallow...but at cap, especially at more challenging difficulties, its not a whole lot of healing. Its going to be dwarfed by your Death Aura ticks and NEB bursts. The neg amp bonus from Shadowfell is going to account for more HP healed than the heal-on-hit effect.
Every little bit adds up, of course, but you might want to consider a more offensive Scion too, since killing mobs faster also reduces the damage you take. Shadow's not bad, but just there are other options on the table too - Arborea, your Spellsword element, etc. Even assuming you're hitting twice per second, at 300 Neg Amp, that's only 28 HP per second.
Darkhallow's a much bigger chunk of your passive heals since it scales on spellpower and neg amp, even with the 1s ICD: at 900 Neg Power and 300 Namp, that's 105 per second. Also 60 Namp, +2 INT and 20 USP is all great value
Plus dont forget about GBoon for extra passive healing, especially since you can just Augment it so its not trading off as much like your Scion feat is.