Well, I think the earlier comment is the most important one here - there's actually a discussion in Summer Knight (when he's going off to meet the Mothers, immediately after meeting the Unicorn
):
"The Nevernever is a big place. In fact, it's the biggest place. The Nevernever is what the wizards call the entirety of the realm of spirit. It isn't a physical place, with geography and weather patterns and so on. it's a shadow world, a magical realm, and its substance is as mutable as thought...it contains within it just about any kind of spirit realm you can imagine, somewhere. Heaven, Hell, Olympus, Elysium, Tartarus, Gehenna - you name it, and it's in the Nevernever somewhere. In theory, at any rate.
The parts of the Nevernever closest to the mortal world are almost completely controlled by the Sidhe. This part of the spirit realm is called Faerie..."
So this establishes that the Faerie courts live in a small, small corner of the spirit realms - the corner touching the mortal realm, where there's a lot of crossover and interaction. However, it also establishes that the rest of the Nevernever is infinite.
So the idea of a former Autumn/Spring court - or even the vague remnants of one - doesn't contradict the cycle-as-written as long as they're not part of it. Four real possibilities come to mind:
- They aren't mentioned because they never existed. QED. (this has the virtue of simplicity, but the vice of boredom)
- The Spring/Autumn courts still exist, but they've been pushed out of the cycle. Some point in the past, the Summer and Winter courts grew enough in power to "close the loop" and exclude them. They're now in exile. They still exist in the Nevernever, but not in Faerie; their strength is a weak echo of what it used to be.
- The Spring/Autumn courts formerly existed, but have been razed. There's no crippling imbalance in nature because there is still the summer/winter transition. The cycle may be more of an oval than a circle now, but it all works out. Maybe you pass through the dark, decayed ruins of their courts when passing from Faerie to the rest of the Nevernever? (a dark interpretation, but certainly not out of character with the ruthlessness of the Faerie queens!)
- The courts still exist - as ghosts. You can find them, reliving their moments of glory, as pitiful shades of what they used to be... Ghosts in a ghost land, and angry over their exclusion from reality and clutching at the past. Their power still exists, but is - literally - a shadow of what it used to be. (This one is my favorite interpretation, and one I'm seriously considering working into the game I'm working on starting locally)