It's likely they're.... well, continent-sized!
Given the writers' general reliance on real-world geography, and what we know of cultures and plants noted to originate from those places, we can judge that the New World is clearly at least North America, possibly both the Americas, and Meracydia may be a detached Africa. (Or possibly the half of it that isn't melded into southern Aldenard.)
There's also likely to be an Australia-equivalent out there somewhere. Does anyone here have the Koala Joey and/or Frilled Dragon minions? Do the descriptions say anything about where they're from?
I think it could be quite doable, especially if you just had the city itself being underground, and surrounding areas outside. Could do some interesting landscapes with potholes and arches. Though I'd like to see a full cave environment too.
For the city itself, larger caverns (possibly with natural light coming in, or lit by crystals) would operate as communal areas, while smaller tunnels and dead-ends would function as "buildings" - walled off at their entrance and used as individual houses or businesses.
For a larger zone set underground, it would definitely need some wider areas where people can move freely. Perhaps some paths that are rather winding on foot, but once you can access flight it "cuts the knot" and you can travel a lot more easily.
Dzemael Darkhold is actually a decent example of what it could be like - if you took away the "dungeon crawling with monsters" thing, lit it more nicely, made it a bit more connected... there are large open spaces and an underground river that flows through it. If you go down a side passage near the start you can look out across a gap to a waterfall, and that's actually coming from the area where you fight the Ahriman boss.
Definitely would like to see something like this!
On your side note about Duskwight cuisine, the lorebook says they use a lot of Mun-Tuy beans, which apparently don't need sunlight. (Not sure if they're meant to be based on anything; the "soy sauce" item text says it's similar to Mun-Tuy sauce but that's the closest we get. Probably completely fictional.) No mention of mushrooms, but they'd definitely be a logical choice as well.
Yes, Ishgardians are a third sub-race of Elezen. They just don't have unique models.
Basically there are two elements to races in this game: there are the programmed character models, defined as specific races, that both player-characters and the majority of NPCs are built from - but also a lore aspect which may or may not line up with that programmed race.
At a programming level, there are only Wildwood and Duskwight Elezen. Ishgardian Elezen are a separate clan according to lore, but there's no need to create a new programmed race for them - they use Wildwood (and occasionally Duskwight) character models, but they do not fall within the actual cultural group that is Wildwood Elezen, ie. descendants of Gelmorra, a separate nation that existed alongside Ishgard.
I wouldn't put any weight on information from the wiki - it's fan-written and dependent on individual interpretation. That said, the lorebook itself is a bit wobbly on sometimes calling Ishgardian characters "Wildwood", specifically in the character 'stats' given for job quest NPCs. But in other places it's clear the terms are used separately - the best example being the racial demographics for each city. Gridania's population is (among others) 30% Wildwood Elezen and 10% Duskwight Elezen, while Ishgard is 70% Ishgardian Elezen. Also, full-prose character profiles usually mention their clan, and Ishgardians are either noted as such or just not mentioned - never labeled as Wildwood or Duskwight, as far as I can see.
There are a few Ishgardian characters that I know have Duskwight models from looking at the character data - Ysayle, Grinnaux de Dzemael (but not his grandfather, who we meet in another quest), Edmont (but not his sons), Jandelaine and his twin brother from one of the Postmoogle quests. I'm guessing it's more based on what works for individual character design, rather than any lore-intent behind it. Ysayle has that rather icy skintone, and the others use Duskwight-specific scars or markings.
Edmont is an odd one though, as he has a customised face and isn't designed with any Duskwight-specific features or colouring - although I did realise his face isn't actually unique but is modified from one of the Au Ra faces, of all things... it's pretty unmistakeable once you can see it. His moustache is part of the Au Ra face build, so maybe the colour of it is still affected by the "Clan A/Clan B" switch and needs to be set to 'Xaela' so it's black. Or maybe I'm just way off the mark and there's no real reason for it.
Re. "that pirate dude", Carvallain, he's an interesting one - but first I have to back up a few steps with a kinda-sorta-spoiler (a missable rather than 'future' plot point, which you might not have picked up on). If you join up the right dots, you can work out he's actually the long-lost son of the Count de Durendaire, who was allegedly lost at sea twenty years ago when pirates attacked the ship. (Once you are aware of that, it makes a lot of interactions with him more amusing. Tataru basically blackmailed him into taking us to Kugane by threatening to let the count know what really happened to him!)
So. House Durendaire. That distinct colouration seems to be a family trait - there's the astrologian guildmaster Jannequinard, and we see the count briefly at the end of Dragonsong. Plus their distant ancestor, who we see in the cutscene of Haldrath's knights after the Aery.
I nearly brought them up as an example in my last post, because despite their rather Duskwight-ish appearance, they're all Wildwood character models - and it's evidently an appearance trait that pre-dates both Ishgard and Gelmorra (but isn't very common in the Ishgardian population). So suppose that kind of trait gets stronger in one part of the population, particularly in that group living in the caves - perhaps it gives them some kind of advantage there.