What are your thoughts on immortals, cliche?
Immortals? Those people don't know when to quit.
How common is it for DK's to have the power of full flight (using wings, etc.) and what sort of regulations must they obey?
The number of DKs who could fly under their own power is pretty small- figure less than 2% of the population. And of that group, only a subset has the time and resources to go through the licensing process.
There are mandatory flight courses that require completion certificates which can take up to a year of weekly classes to complete. Most of those classes consist of desk work, with a handful of test glides and flights being held in specialized gymnasiums with safety nets installed. In addition to Central City fees required to register and take the courses, teachers charge variable rates for their instruction time.
After the expensive and time-consuming process of getting a flight license, the flier must then observe all Anduruna regulations. When inside the city walls, they must use predetermined takeoff and landing sites, and log their departure and arrival times at those locations. An automated system will coordinate their permitted flight times to ensure nobody collides with anyone else. Although with the number of fliers, this has only happened a handful of times in history even during the unregulated days. Registering a flight plan incurs a small administrative fee. There are also fees for any items being logged for transportation, and dozens of rules corresponding to garments and approved satchels or packs for carrying items of various classifications. You can't just carry something all willy nilly.
This means that most fliers are either wealthy, or they have to be serving as specialized private couriers to help justify the expense of flying.
The result is that a lot of licensed flyers will use the takeoff and landing sites- since it's rather obvious to see when someone doesn't- but they'll often skip the flight plan login and fee, and just fly off. It's not enforced very closely, except by other, more zealous, licensed flyers. So it's usually possible to fly whenever you want as long as you use the predetermined takeoff and landing points, and don't run into a snitch.
Outside the city limits is another story- it's still illegal to fly without a license. But if you have one, you can soar essentially anywhere you like as long as you don't leave the overall Anduruna region.
What districts are our guard trio originally from?
Bill grew up in a more mainstream region of Theophanies, Damon is from Norvondire, and Woods is from the farmland just outside the city wall to the south.
What made Vi abandon her fashion dreams? (i suspect this is plot vault)
...Plot Vault confirmed, sorry. ^ ^;
How long can Nightmares live for and what happens to their bodies when they die and respawn?
Nightmares can live anywhere from 10 years to several centuries, based in part on the sort of body they are capable of spawning. Once they die, their spirit is returned to their plane of origin, and the body at that point is just decaying matter like any other dead body. It has no relation to whatever they spawn into next.
How does the Troika determine Power Intensity ratings (is it "eye-balled", is there an objective measurement or a test, etc.) and are non-destructive utility Powers (such as Bobby's light orbs, Nainso's "ignorance field", and Whip's autokinesis) automatically ranked lower on the Intensity scale or is it possible for them to be ranked as high as a 6?
They tried developing objective measurements, but due to the totally differing nature of some powers, realized that they were dumping way more time and effort into categorization and measurement than the utility they could potentially get out of it. So the Troika basically eyeball powers for rating.
They started measuring powers based on general combat effectiveness. If one guy with no training could potentially take out a fire team of shock troopers singlehandedly, they'd peg his power at a "2." Combat effective, formidable, but generally not enough to handle an entire platoon alone. Each rank above that is another order of magnitude higher, so the progression isn't linear.
Nondestructive powers are just pegged onto this conceptual scaffolding, even though they don't necessarily cause damage. Take Nainso's ability, to direct focus. At a level 2, he can direct the focus of a handful of characters rather effectively, especially if they don't notice the influence. If he were a level 6, he could murder someone in the center of a crowded stadium, while every person was obsessing over a sleeping ryuu-neko on the sidelines, and nobody would ever notice or remember him.
So it's a rough measure- not used to precisely categorize anything, but serves to give Troika commanders a quick 'n dirty impression of what sort of punch a soldier might be packing.