From what you say though, my Kemmlerite should have at least 2, if not more, contingency plans in place to ensure that even if he can't seduce the White Council PC into necromancy, that he can still complete his ritual, and if not that, have a couple plans for escape.
This sort of planning is most important on really long lived folks who also have experienced a lot of life and death situations. So Mavra, a rare black court survivor, is shown as thinking really hard about Dresden when her scourge comes to town. She fills the place with mortals, arms them with a variety of things difficult to defend against, puts out a decoy to "die" and actually spends the whole fight veiled and upstairs, taking pictures, so all the really destructive things that occur downstairs don't touch her AND she learns even more about Dresden and his allies - which she uses later to get her hands on Kemmler's book and eliminate anybody but Dresden who might have read it....
That's a very convincing portrayal of an intelligent survivor, maximizing her own abilities and limiting the scope of her enemies, using mostly resources she doesn't care about.
So if your Kemmlerite is somebody like that, someone who survived perhaps the White Council war on his former master and the hunting down of disciples, he's not going to try to seduce a White Council wizard without a damn good backup plan.
OTOH, there has to be a pretty big payoff to risk involving a WC wizard, so his alternative to do the darkhallow-thingie must either also be hazardous or it'll be less effective or, if neither, he's getting big fate point compulsions from aspects to involve THAT wizard in spite of an adequate alternative....in which case he'll be doing the alternative with a big wad of fate points in hand if the WC wizard does try to interfere.
At a bare minimum, his escape plan should be solid, or it isn't credible that he survived this long. A perfectly adequate precaution is to not seduce the WC wizard in person. The simple expedient of a phone call, or a letter writing campaign helps quite a bit, although they have the downside of being liable to interception, so maybe the mail drops off a stone and ritual instructions for the communication spell. Or you work through surrogates that you can afford to lose, etc. A WC wizard is mostly dangerous in line of sight, or with a connection to you that you don't know about. Don't do the seduction with either of those things available (if using a ritual stone for communication, rig it so if very much power goes through it fries the connection, etc).
Now to kill two birds in one stone, have your flunkies be signaled when the WC wizard contacts you, so while you're doing the "come to the dark side" seduction, your minions are doing the apartment complex raids etc....ensuring that wizard won't interfere with minions not up to his weight class.
Red court noble has another problem - they tend to be long lived and cunning. Again, best way to deal with a Fist of Yahweh is to introduce the venom while you are nowhere near the situation. Spike his drink or his food, or send some flunky who has displeased you to try to introduce the venom addiction....I'm dubious that the venom would work very well on a True Faith type character given red court catch, but maybe the Noble's trying under the principle of "you never know until you try".
One payoff for clever PC actions is to maybe get in the same room with either of the manipulators to force a physical confrontation, where normally neither would get caught out that way. Eg, the WC wizard insists on personal instruction out of a claimed fear of wardens and an insufficient skills with magical detection and veils to ensure privacy without his "mentor" helping, then sets up some kind of ambush. The Kemmlerite then might enter a hornet nest of PC nastiness, but still should have a lot of routine items/potions/thaumaturgical aspects and basic all purpose escape and combat plans for ambushes.....I dislike "he escapes in black box text" type fiat ambush failures...I want it to fit the mechanics of the game. DFRPG/Fate is easier than most because of the concession mechanic, but it still works better if everyone can believe their character would fail to catch the bad guy via the rules of the game, however expressed.
What I have found satisfying is when somebody like that has a realistic set of contingencies for an ambush, ANY ambush, not just the PC's (just look how much better Dresden got after a few years at war). If caught in an ambush, a character like that doesn't fight. He runs (see Nicodemus, pretty much always when at a disadvantage). If the PCs have a really, really good plan, maybe they learn some cool new tricks for their own use later, and if they can compel the right aspects they might be able to keep the person in one place long enough to take them down...but it should feel like a hell of an accomplishment, and PC's should end the fight feeling like the dog that caught the wildcat...