Faerie question! What do you think about the relation between their realm and our geography? Your 'white American fae stories' mention reminded me that I was thinking about this a while ago - if someone is interacting with ~fairyland~ from a regular ass non-European continent, is it (Watsonianly!) weird that they are interacting with Welsh-culture faeries? Or is that place decoupled from our land? Conversely, if the supernatural world you access depends on your human world location, does that imply that different cultures' supernatural entities have a similar geographic relationship with each other to human lands? or does it imply that the shape of the fae is downstream of human culture a la Small Gods?
Oh this is FUN and I am going to RAMBLE
So the thing that sort of answers and sort of complicates both of your questions is that Welsh faerie lore and mythology, while having some Venn diagram overlaps, is nonetheless Very Much Different from Irish or Scottish (or indeed English) stuff. It's an interesting one, because while I have increasingly strong Views on the way Welsh faerie lore is used by white American authors who want to write about elves with wings who fuck and think this is the solution, actually a huge chunk of what those authors use - and what Americans in particular more broadly know about faeries - isn't Welsh at all, it's Scottish. Seelie and Unseelie courts, season-based courts, never thanking a faerie to avoid offending them, selkies, the list goes on. None of that is Welsh.
I, as you know, have been writing werewolf erotica, for fun and sport. Set in Wales, of course. I haven't directly included faeries yet, but they've been mentioned, and I know how I'm going to be building that part of my world. And to me, faeries come in different species with a different geographic distribution - if my characters were to approach the Fae in Wales, it would mean entering Annwfn. They would meet very Welsh types of faerie. Welsh rules would apply.
If they were to go to Scotland, they'd be dealing with different types entirely. Seelie and Unseelie would now apply, and not thanking and all that jazz. To speak like an ecologist for a moment lol, it's a question of biogeography.
Soooo, yeah, I find it weird when American fantasy lit describes Celtic fae creatures in America, because to me... surely there's native shit there. Like what is this? Did the Fae colonise with the humans? Has the American Otherworld been invaded and settled? What am I looking at, here? Why is the author ignoring this question? How are there gwyllion in those mountains and what did they displace? Did they follow the people and just naturalise, or are they invasive? Are there gwragedd Annwfn in Lake Superior? How is that working? These are questions I have, but alas, no answers.
(I can allow arguments for Appalachia, given, you know. <same-mountains.jpeg>)
That said, the rules are fuzzy for time/space distortion with entering Annwfn. This is a (relatively) new addition to faerie lore, because once upon a time Annwfn was a place you could just... walk to. It had a geographical location, like Rhyl. You could find it on a map, and that map would tell you it was Somerset, pretty much. But over the centuries, human population density grew; Somerset stops being a place of mystery and starts being the place your flighty cousin ran away to and now grows a cider orchard. The magical realms hidden in thickest forest are demonstrably not there when you cut the forest down and just find a bunch of exposed bears. So the lines, as it were, get redrawn - we know it's there somewhere, but part of the magic hides it; so maybe what we were pointing to on that map wasn't Annwfn, but the doorway...?
By the 1700s at least, the concept of the faerie ring being a doorway between worlds was fixed. The 1800s gave us the Victorian concept of the veil between worlds, two worlds overlaid on one another, which mapped beautifully on and basically reconciles the issue perfectly in the minds of believers (faerie belief in Wales persisted into the early 1900s). Enter that cave and you'll enter Annwfn; not because it's in the cave, don't be silly, it's because the door is in the cave. Step in that mushroom circle and see another world; not because Annwfn is a patch of land three feet across in Mam-gu's garden, don't be daft, it's because that's the way through. Welsh faerie myth was already enamoured of the time distortion element, but this is where spatial distortion kicks in as well.
So understanding all of that means you can exploit those rules to explain a lot. Watsonianly speaking, does a doorway lead to Annwfn if it's in Wales, but Tír na nÓg if it's in Ireland? Or could a doorway for either turn up anywhere, given that spatial distortion? What is it about Wales that ties Annwfn here specifically? Does the land generate the specific type of magic needed to fuel it? Or could it feasibly go anywhere now, as that separation between worlds has evolved - initially they lived in this world, but they evolved to straddle here and another, and then to draw a veil between the two, and now they run parallel and so can send the tunnels between the two wherever they like. It depends on the story you're telling, I guess. As I say, I know how I'm doing it lol! But there are options available