What difference did the Twelve tangibly make?

[Wix]

Ishgardian Elezen who use Duskwight models aren’t Duskwights. Duskwights and Wildwoods only exist in Gridania but they never made separate Ishgardian Elezen models so they share.

The whole Ishgardian thing is sloppy, and people will never agree on it, so I won't directly address it, but I'll point out that there's at least one Duskwight-model Elezen in the Shroud who was also retconned to be a Wildwood (Mother Miounne).

Duskwights and Moon Keepers are fairly prominent in Gridania storylines

They're really not. You probably remember the Lancer questline, and the two Duskwight NPCs in Gridania ("Damnable Duskwights!" and the airship ticket guy), but go do the rest of their quests and you'll see they're not prominent at all. In fact, they're pretty rare flat out in ARR onwards. There's a handful who are nameless and will briefly show up for a one liner in a delivery quest, but beyond that, it's basically two Duskwights per zone, tops, usually one, sometimes none, outside of the South Shroud and Sharlayan.
I'm not even joking. Finding the token Duskwight in each city is like playing Where's Waldo.

they only exist in The Black Shroud

In theory, but not in practice. In fact, the most prominent Duskwight family in the game actually had a seat on the Syndicate, and were located in Ul'dah, but SE made a point of killing them off going into ARR, though the son survived. He gets a brief cameo in post-ShB questing and a holiday quest you can no longer access, which makes him the only Duskwight in the entire game with a voice line. You can still pick up vague references to the family whenever alchemical things come up in Ul'dah, or their abandoned mansion in Thanalan that a Lalafell gang moved into, but that's about it.
Ironically, outside of certain parts of the South Shroud, the place you'll see Duskwights most prominently is among Garlean forces and Sharlayan groups. There's also an argument to be made that they existed in both La Noscea and Rabanastre, but those are yet more instances where it's messy and we don't have enough information, so people will argue whichever interpretations they like.

/DuskwightGrievanceRant
*cough*
Buuuut yeah. The conclusion of MotR wasn't exactly what I was hoping for either.