C. Why haven't they changed into mindflayers? Why haven't you showed any symptoms? Why can you use the tadpole, to do things and not change? (A new source of power for you to exploit? These people know lets use them, help them, get in good with them, learn their ways and then discard them.)
D. They offer you a way to utilize a new power per C. by fully understanding what is going on. Your so bent on getting rid of this tadpole (a thing a good person would want to do, or one who is fanatically against mind flayers.) why if it's not turning you into a mindflayer, then it's a source of power. A source these leaders seem to understand, so you can use them to find out about it. Then of course they have to go once they are no longer useful.
I think the key to why I think evil Drow wouldn't necessarily go illithid route is here. To be given power by others in their culture is the same as being offended, just think about the way they treat Dryads. They are not only evil, they are very very prideful and would definitely hold a grudge to being kidnapped and infected by a parasite. From there its not a big leap to want to completely destroy the Absolute cult and make allies of convenience if I have to regardless of their alignment.
