I can't really answer all the questions you pose, but in the little I am familiar with the system at Universal, people with the pass are allowed to do something else while they wait.
A significant difference between the two properties is the difference between Express Pass and G+/ILL. Universal came up with an option that is simple to use and easy to understand. Disney came up with a convoluted system that is a confusing mess, with a high degree of uncertainty.
Disney parkgoers are frustrated, and, as we often see, angry people want a scapegoat. I am inclined to think Disney's version of Express Pass is 90% of the problem. It is horrible.
I can say with EP, park goers just go on whichever attraction they want. Unlike Disney, Universal does not limit where EP users go while their park is open.