Alicornutopia

It is my current understanding that the events of Visitor are noncanon for any story set in that world with a Promise in it (or at least the visitor doesn’t accomplish anything much before Promise does, most of the time), but that the events of Queen are canon for Visitor and all the Fairyland sandboxen. Is that correct?

Queen is canon for everything in the setting. Visitor (the character) and Promise are incompatible.

How... weird Fairyland can get?

hum, using examples:

Can you find "candylands" in there? or maybe... oceans of milk or honey?

places without regular geometry? Places with some magical properties like a healing foutain or something?

how giant "natural" cities? I understand there ought to be regular cities somewhere with ever growing populations, but naturally occuring cities are a thing?

Well, Effulgence has the emerald and sapphire seas, which I’m guessing *really are* those gemstones but liquid. The original Visitor had a liquid gold fountain. I would be extremely surprised if there was no healing fountain somewhere within a month’s fast-flight, or oceans of milk and honey (or possibly other sweet things, since mortal foods don’t seem to exist). I would be surprised if there was non-Euclidean geometry, though.

Candylands and animal protein: unlikely. Honey or something similar: possibly. Geometry is strictly euclidean. Fountains seldom if ever heal people. Cities do not occur naturally. IIRC kappa means for the seas to be floored in sapphire and emerald more than liquid instances of same.

The seas are made of water, but floored in and coloured like emerald and sapphire.

So it is just a old boring ocean (with more fortune per square mile than someone could spend in a entire lifetime)

My questions are meant to identify "edge cases" of weird things that might crop up.

Fairy powers... does rarity relates to how powerful/useful it gets? Are they always a single thing per fairy? Stuff like Sherlock-species "one sense type" and Promise's "one immunity type" that vary from individual tend to be common?

Rarity is loosely correlated with usefulness. (In particular breeding kinds tend to have worse powers than plural starting kinds tend to have worse powers than one-of-a-kinds.) The "one X to an individual" thing for plural starting kinds is not universal but it's apparently happened in more than one kind.

Not sure if that was mentioned anywhere, but I am assuming breeder kinds can breed with other fairy types (not necessarily all, but at least some) because as far I can remember breeder courts always seen to have a single leader...?

Multi-leader situations of any court tend not to be stable. Somebody gets and keeps the upper hand, whether immediately or over a few years. This doesn't mean that they can breed with other fairy kinds, although some can.