I'm not an accounting guy, so I can't answer most of your questions. But I have taken several courses in the economics department, and the Accounting PhD students also took some of those courses with me (like Quantitative Economic Analysis, and Econometrics) as well as some courses I didn't take (things like Microeconomics, if I'm not mistaken). So, at least here it's possible (and common) to take classes in the economics department.
And from my experience so far, each PhD student follows a different structure regarding coursework. Even if we are students in the same research areas, our research interests are different, and then we take different courses, for example. Maybe that's one of the reasons many universities do not make the coursework structure clear.