On B, I'm confused on why you are using <-1, 1, -2> as your direction vector. If the line is through D and perpindicular to the plane of the triangle ABC, why not just use the normal to the plane as your direction vector? Perhaps I'm not seeing something in your reasoning.
For question C, we need two vectors (create the normal) and a point. You have one vector in the direction vector of whatever line you made in B. You also have the vector AD (or DA).