I don't think these are any issues that need to be addressed. Dark Arts helps DRK actually stand out from the other tanks, and feels good. If your MP management is good you get to go edgelord and power up something.

Your idea for Scourge kind of already exist, but SE decided to lock it behind PVP. Allowing Sole Survivor to make the target weaker would make it not only more useful but more deserving of its CD.
Salted Earth doesn't need heavy; its the strongest ground placed AoE in the game. I wouldn't mind if unleash had its PVP effect as well, but since its out main aoe enmity spell I doubt we'd see its use in pve content.
Your idea for Parry needs to be universal. Dark Dance is honestly good as it is. It really shines in dungeon content with Dark Passenger for a ton of dodging. We just need Parry to actually scale better and have more use. That 20 percent should be what pushes DRK to a near perfect parry rate if we prioritize parry.

I personally believe the biggest and most important change DRK could get is on Grit. Grit holds this class back so much. Without it we lose out meager sustain, with it we lose our TP and damage increase abilities. It needs to get rid of those gates.

The only other small change I would love is to get rid of the "outside sources cannot replenish MP while under Darkside".