If no dwarves are ever above zero stress, then the problem isn't solved...
They're living in relative luxury, so it doesn't seem like a bug to me. There's abundant food prepared by a legendary chef, masterwork silk clothing, half the rooms have masterwork furniture of one from or another, there's a library full of books, a well stocked hospital with plenty of soap for baths, and every time a goblin siege appears the goblins get doused in magma long before any dwarf has to look at them. The biggest problems in their lives are boredom, cold weather, and occasional bits of miasma when some claimed food goes rotten.
The problem with the old version in my opinion was that some dwarves would be virtually unrecoverable regardless of how nice you made their lives. Stress should be an incentive for the player to improve the living conditions in the fort. Randomly occurring, untreatable depression just isn't fun (or even !!FUN!!). It would definitely be a problem if they were happy despite living in squalid conditions surrounded by the rotting corpses of their friends, but that's not the case in my current fort.
It seems that such dwarves are basically unsaveable, unless you're willing to make sure they never come in contact with another dwarf ever again.
I'd be interested to see what would happen if you actually did that. Isolate them in a burrow behind a locked door, dropping in alcohol and masterwork meals via a hatch or airlock, and give them as many sources of happiness as possible. Would they be able to function normally or would they be stuck in a constant loop of trying to meet with the mayor?
Also I wonder if it would be helpful to segregate dwarves based on their personal values. Would they stop getting into arguments if they were living with a dwarf who shared exactly the same values and beliefs, or is "in a constant state of internal rage" guaranteed to make them argue?
EDIT: I've just checked my own fort and I've got three adult dwarves with anger propensity higher than 90 (constant state of internal rage). Only one of them has any bad thoughts from getting into arguments and he's also the most stressed dwarf in the fort (I take back my previous statement about none of them having any significant stress). There's a different dwarf with an anger propensity of 64 (quick to anger) who actually feels satisfied after getting into an argument. I'm trying to figure out the difference between them but nothing obviously sticks out.