@eeky ,
I believe he's saying it's a causative (if I understand correctly what you mean by causitive), but that it's not "Emily" who "let Professor Y sit there", but the person who asked Emily to move. Though perhaps it could also be read as "Emily's moving seats" is what allowed the Professor to set there.
I'm more interested in the first part, actually (here's the sentence repeated)
そして、エミリーの座っていた、床の間の前の所に、Y教授を座らせた。
I assume there's something implied after the 座っていた, such as のは or から or something with a similar effect... but I'm surprised that here エミリーの座っていた apparently implies that she _changed_ her seat... I'd have expected that it would need to be something like 座り替えた to imply that she "re-sat", otherwise, couldn't it potentially be referring to her first act of sitting (ignoring the fact that this wouldn't make sense in context)?