Okay, forget everything I've proposed and go solely on what Aurora says in SK.When you read this, what are you interpreting "Summer" as? I'm interpreting it as the Court.
When I read this, I interpret it again as Aurora saying the Summer Court is at peak strength. Which implies their strength, their power, ebbs and wanes with the seasons.
I agree with both of those, where I differ is in the idea that the Summer Court she's talking about is the sum total of Summer on Earth.
I think you are picturing a single Ball of Power that floats back and forth from Summer to Winter and Back, so when Summers power is Waxing toward Midsummer and Winters in Waning, it's because Power is moving from one Season/Court to the other; Summer is stronger at Midsummer because that's when they hold the Largest Share of total Season-Power.
I Picture two separate but equal Balls of Power (Muspel and Nifel, per Norse Creation, but I digress). When Summer's power begins to drop, that Power (which per the Mothers cannot be created or destroyed) is slipping to the Southern Hemisphere in equal Measures (because that's how Seasons normally work across hemispheres), and to counter that Winter's Power is moving up from the Southern Hemisphere in lockstep.
You are picturing one ball being passed back and forth, Im picturing two separate but (nominally) equal balls of Power that are being passed Up and Down with opposite timing.
The linchpin of all that is the WOJ on there being a Single Stone Table. If it were as you describe and
Only the Northern Hemisphere matters, that the Southern seasons simply do not factor in, then A)the Table would never move, and B) the relative politics of the Wyld would not matter.