I would prefer absolute world coordinates, plus relative coordinates from the last position (if I am moving something.)
This should be adjustable since there is no one way that everyone should do CAD. Implement all three of your methods, and allow a config screen to switch between them.
Another thought-- can we use scripting in the UE4 Editor to modify its behavior, just like Blender can be extended by python? Perhaps it could be done in a plugin? I don’t even know how much I don’t know yet !