So, can דיון mean any logically constructed defense of a proposition, as the English word argument does, or id it limited to something emotional or hostile?
It's the other way around. The emotional and hostile one is ויכוח. The word דיון refers to any discussion, and additionally to a hearing in court.
It's funny because in spanish (I'm native) is the other way around. "Discusión" means an hostile argument, whereas "argumento" means a rational defense of a position, maybe in a discussion:-)
An argument means two things in English. A presentation of an opinion and a discussion. דיון only means discussion.
The other one is טיעון.
In court you have a דיון, where each lawyer presents their טיעון.