I would almost swear I read that the 2D design (with delta-wings, but the compression ramp was wedge shaped) produced lower overall drag...
So a highly-swept design is more efficient?
I think, this is actually quite simple geometry. A Circle will always have less circumference than a Rectangle for a similar area. So, if for any shape where the other types of drag are equal, the tube like shape will have less parasitic drag, as it exposes less area to the airflow. If 2D design were more aerodynamic and flight efficient, bullets would not had a round shape now would they?

The main advantage of using 2D Nozzles is in heat management and better easier LO integration with the rest of the design.
A highly swept design has nothing to do with the above argument.
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Ability to change the direction of the exaust trust without moving parts. X-36 had such technology on it.
Assuming that's not clasified, how the hell do they do that?
I hope you run a google search before you asked that. If not try it and check one of the many articles on the subject. It's not rocket science but its not a short explanation either.
And please keep your questions relevant to the topic, or create another one after you have made the relevant search.