What does the sky look like? I know the elements move across it in a seasonal pattern, but is that a band across the apex or horizon-to-horizon, north-to-south and east-to-west?
Horizon to horizon, unless you are facing sunward, in which case they fade into the brightness/darkness. Recall that the elemental planes form the cosmological shell of the universe which contains Pellatarrum and are therefore larger than the material plane.
Are the elements in the sky clearly visible or background visible (like the moon during the daytime)?
It rather depends on the element and the time of day. I think I need a sort of chart here...
DaytimeEarth: Much like our world's moon, only with more variation of color and shadow.
Air: The deepest, bluest sky you've ever seen, perfect for losing yourself in.
Fire: A haze of pinks, reds, and oranges, rather like seeing a smog layer at sunrise/sunset.
Water: A reflective, rippling layer, like the sea on a cloudy day, only upside-down.
NighttimeEarth: Mineral deposits and strata sparkle with light conducted from the positive energy plane, giving an impression of a fiber-optic aurora, as different materials catch and reflect different colors.
Air: Lightning and other atmospheric effects, including the light from various cloud castles, creates a slowly-moving starfield of incredible depth and complexity.
Fire: The Negative Sun helps bank the fires somewhat, reducing the flames to the cherry-red glow of hot coals. The City of Brass shines brighter and hotter than its surroundings, much like a Full Moon.
Water: The sky above is dark, mysterious, and capricious in what it reflects. It's rather like being on the water just before a storm, not certain if it will smash into you or pass harmlessly overhead.
Nighttime Light levelsStart of Winter: Waning Crescent
Midwinter: New Moon
Start of Spring: Waxing Crescent
Mid-Spring: First Quarter
Start of Summer: Waxing Gibbous
Midsummer: Full Moon
Start of Autumn: Waning Gibbous
Mid-Autumn: Last Quarter
I've seen references to a Nightspire in the comments, which appears to have been destroyed. Is its previous existance a world secret (only the most learned know of it and maybe not even them) or generally known?
It's existence and subsequent destruction is unknown, more from apathy than from any desire of secrecy. Only the Dwarves care about its existence, and that's mostly as a footnote in the original Blueprints of Creation.
I haven't yet decided if the Dwarves know it's been destroyed or not.