Alternatively, an equally unscrupulous but smarter ritualist could tie up a hundred people in his den. Then he could ritually bleed or torture them one by one for one exchange, inflicting small amounts of pain each time - a mild consequence. By the time he came back to torture the first victim, an hour (couple of scenes) would have passed, allowing them to recover (the consequence to heal) and the circle to start anew. Doing it for a few hours each day and then having some helpers water and feed the captives would allow thousands of shifts a day.
This one doesn't work. Consequences require more than just time to heal...they require actual justification (ie: medical attention). Something similar might work, but not on quite the scale you're talking about. Everything else is mostly true though.
Also, have you read Changes? Because the Red Court totally do exactly this, though they can't afford the attention drawn by doing it regularly.
And finally: With as much time and effort as a ritualist needs to put together this sort of thing
1. Someone definitely has time to stop them, at least potentially (particularly if warned by, say, the Gatekeeper).
2. A good Wizard can get almost as many shifts with pure ritual prep and no sacrifices at all. You can do 20 or 30 Declarations an hour if you're really good (Yes, really). Sacrifices are for people who are lazy or in a hurry and have easy access to the people in question, not really this kind of thing.
And doing this on the kind of scale you're talking about will also draw the attention of, well, everybody. Expect White Court and Knights of the Cross alike to show up to stop you. What you're doing is both profoundly wrong and stupid for the supernatural community as a whole in the long run. One of those two things'll piss off almost everybody. Hell, something like Ferrovax or Odin might show up and swat you like a bug for drawing attention.
But yeah, disbanding the White Council without establishing a different magical law enforcement body immediately is the definition of a bad idea.
They are not casting a spell. They are preparing a ritual, which is often measured in multiple scenes. The rule is that Ritual-time, for every scene you could have been involved but chose to do the ritual instead, you get a scene of ritual preparation. The simplest way to use that time is research, giving a flat +1 per scene you give up but you can do other stuff like ritual torture or ritual sacrifice or blood magic or calling your sponsors or additional casters to help you raise energy.
Besides, thaumaturgy preparation does not have to be done all at once. The Red Court killed hundreds (if not thousands) of people over many days to produce the bloodline curse.
The actual casting of the ritual comes after you have raised all the complexity to Hell and back-say, 6000 shifts for Tunguska. Then you must actually call that much power and fuel it into your preparations. Assuming Ebenezar had a Discipline and Conviction of Superb back in 1909 and a +3 control bonus total via foci and specializations for Disruption, he can safely call Power equal to his Control-4 at every exchange. Statistically, once in 256 rolls you'll get a -4 hence playing it safely. So Ebenezar calls 4 shifts of Power at every exchange safely, needing 1500 exchanges to get it all. An exchange is a few seconds so Ebenezar would need to actually cast for 2 hours straight to get that much power.
Those two hours are the actual casting. You can't stop once you begin or you face fallout and backlash... of all the power you've gathered until the interruption. And several thousand shifts of backlash might be quite spectacular.