I don't contest that it's nice to meet NPC's from canon material, but I still feel all of your questions should be asked retro actively.
Specifically in terms of the players you want answers to questions that the players may not know the answers to until after they have been asked/ played out.
That is, my character may not have an opinion on Tribunal issue X until it's brought up at Tribunal.
What I should have clarified is that working on the fly should be matched with 'retroactive explanation'.
What I often do is grab the nearest thing to me that has a name and that's how a character gets named. The shoemaker is, (grabs a Cd) "Jean" Williams.
If the NPC has no merit, he's forgotten. But the characters need this shoemaker I start thinking about him more. Ok, why is it bad to screw the shoemaker? What is the reprocussions of asking his daugher on a date?
These questions are a waste of time if no one pursues the daughter, but only after a player makes the action does the question have merit.
In the case of your questions I propose it's much simpler to answer these after the fact.
Prep the Tribunal by picking NPC's and giving them generic House positions. The Tytalus want a fight, the Jerbiton want order, ect ect.
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- Various magi's positions on formation of the Lotharingian Tribunal. Who is solidly "yeah" or "nay"? Who are swing votes?
I think it's much more interesting to discover the players positions as they make them. Furthermore, the players, having made them, may seal their futures fate without realizing it. Let them play it out, have no idea what they are going to do. After the decisions are made, shape material based on what they did. It's much smoother and less forced/linear.
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2. Which magi have been targetted for infernal interference of their opinions?
Again, after the players make their decisions examine what they have done, what they have chose, why? Now you have material that's played out, that's 'canon' and not theory. Say the demon watched them, observed them, picked his target. Path to Hell is paved with good intentions right? Again it's much more natural to have the characters actively walk a path than to set them up for damnation. In reflection, they will be hard pressed to argue you as GM tricked them.
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3. Which noblemen are being manipulated by infernal minions?
A large cast makes it difficult for the GM to handle everything. A)Take your vauge sketches of characters and hand them out to players. B)Let the players make up people that might be there, such as noblemen.
This is an important thing. There is the written character, the character 'imagined', and there is the character played. I have certain players, that by their nature, no matter how evil a character you give them, they will play someone that is noble. Same for ineptness. You could make the perfect character, and I have player that is simply always a 'clumsy' character.
For particular scenes, or even an evening, let the players play someone else. It's refreshing, it mixes things up, and it lowers your work load/burden. After the session, consider the natural choices they made. No preplanned agendas, no mission requirements, no 'imagined' characters. All there is the game that was played.
Take this material and look at and retroactively plan based on it.
The Baron was an ass? Maybe that instill anger in the host? Maybe a demon can exploit that anger, alter it to wrath.
Again, I consider this a more natural course of events than preplanning.
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- How will this affect their relations with the covenants in their domains?
Again, I still think a retroactive question. Let the material play out. Sit down with your friends with the barest of material. Just play, just game, just let the players be the characters. Then afterwards, look at the material they handed you, the material you should see as potential, and find your master plan inside it.
My gaming troupe was been gaming on and off since 2nd edition. We've got 15 odd years under our belt and one of the few things I feel strongly feel about GMing is one should never over work themself.
This is much longer than I anticipated, but hopefully you can find something here that's of use. It may be we have different styles, but I see so many GM's feel real grief and anger for work that PC's simply do not appreciate. I'd like to spar any GM from work that isn't needed or not appreciated. I think a combination of ab lib and retroactive explanation is the secret to a natural, non-linear, long lasting game.