See, the balance to those massive rituals is narrative, not mechanical.  If a player is using a massive ritual there’s no reason not to impose some sort of time constraint that will prevent him from breaking up the casting into X rolls that he can’t fail.  Alternatively you could an assault on the party during the casting, depriving them of their artillery while the sweating caster desperately tries to finish the spell in time. 

However, since scenes have variable length, it becomes hard to assume how long hte spell caster is out of play for.

Well, it's true you wouldn't know ahead of time, but since there's no such thing as a non-narrative scene it would be the next 99 scenes the group plays.  My group probably averages about 5-10 per night, so say 15 gaming sessions?  Not really doable.

A couple thoughts I had while reading your post:
1) You say that the reestablishment of Spring diminish Winter, but wouldn't it also diminish Summer?  Or maybe you meant more tangible assets like territory in the Nevernever and Fae who swear (three unsuccessful attempts to spell "allegience") fealty to Spring.  Would Spring, and presumably Autumn, draw metaphysical power from the same sources as Winter and Summer?
2) Would events arrange themselves so Autumn returns as well, to balance the power of Spring?  If not, would that mean there's a lot of power sloshing around that other parties could take advantage of?
3) I don't know how obsessively you read WoJ, but according to Jim the Erlking's origins are in Summer, but he represents the dark counterpoint to the otherwise warm/light way that Titania's court is portrayed.  You could call him the Summer King.  Similarly, the Winter King (Santa Claus, no joke) represents the less harsh elements of Winter.  It's quite possible that, should the players be assisting Spring they'd find the Winter king helping them, while the Summer King worked against them. 

Count Immunity as level 4 of toughness?

It could also have to with natural strength of will.  For sure it takes more than just age, "It's My Birthday, Too" features a BCV who's capable of creating the zombie-like spawn and was turned less than a year ago. 

Usually I’ll come up with some enemies, some antagonists, some potential allies, and some color characters.  I’ll give the important NPCs goals, figure out where those goals will go absent PC intervention, throw a couple clues at the PCs, usually coupled with compels, and then go.  If I can get away with it I’ll come up with the final set piece ahead of time, that’s usually easier if the set piece involves some sort of big ritual that is specific to time and place. 

I also did a lot of city creation, so I’m rarely caught without an interesting venue for a conflict.  Depending on your definition of “winging it” I may not do so at all, but I rarely have a predetermined plot line that the PCs are expected to follow.

My primary thought processes are “how can I screw up their characters?”  Not in an “open season on PCs” way, but in a “how can their lives become more complicated” way.  Thus far I’ve forced the White Court Vampire to confront the monstrosity of her own actions, had the Knight of the Cross question his calling to the point where he hasn’t even drawn his sword for three sessions, and had the poor Wizard end up looking at some truly powerful Outsider magic with his sight.  I was particularly proud of that last one.  Oh, and my other big concern is “how in the hell do I motivate both the Knight and the Succubus to be interested in this?  My player decided she wanted to be Lara Raith’s illegitimate daughter, so in a pinch I can use “Lara’s Inscrutable Plans,” but I don’t like to lean too heavily on it. 

Maybe do away with uses per session for items made with Greatest Artificer entirely and have the item make the attack against Discipline every time it’s used?  Also set a hard floor of Great for the attacks, so there’s always at least a fair chance the user will have to either take stress or spend fate.

Ah.  Probably a ton, but zompires would probably be the least of their worries. 

Of course, now I'm considering the possibilities inherent in a BCV plague.  I wonder if BCV "spirits" are usefull in a Darkhallow, or at least a certain kind of Darkhallow.