Back to work, you!
Possibly, but then you'd have to contend with him actually not having a mental lapse in the middle of casting. At the same time, why not create the mental construct and begin instruction once it was complete? It seems a rather foolish gambit to play when one has already invested. I suspect the vision may be a bit of death (MC got better) and speaking via that means. Things Marek has learned. Or in a near death state the construct of Marek's image by another force, which would quite possibly be Lilith, to communicate to the MC. A random woman appearing may not be the best thing to have occur, you know.
As for the body... illusions would be required to be kept up for a prolonged level of time. Plus you'd most likely have Lilith, in that scenario, preparing other things in an attempt to remove said sister from a position of power. Mage-on-mage violence does tend to get messy. Unless you're gunning for a grand conspiracy that involves Marek, his death, and Lilith. But you'd have the crux of explaining away why, in all this time, Lilith did not obtain the book from Marek and utilize a grander plot via illusion to oust the sister.
While such things are possible, I would denote a bit of pragmatism and storytelling flaw from that; the overuse of magic to make scenario's fit. Even the invade mind spell has limits and natural resistances. Thus things like illusions would also have limits. At this point it is all speculation... and our dear Dev may be using it to help further flesh out the story.
As I recall, the scenario of possible NTR always begins with a rather tame situation in which the character is able to interfere and quickly end. Each progressive step allows for the player to continually intervene. While it is honestly outside the structure of the character created by the dev, I have wished I could use that for more leverage against one or both of the other characters. But then again... I am rather Machiavellian in such situations. Why I had a large interest in The Tyrant when I first came across it. But the lack of actual execution on retaliation or utilizing it to one's advantage was never implemented.
