I work on the ground plane too, sometimes, when I do 2D design (parallel projection + top view). So, as I understand, you basically want the middle button to work as Pan instead of Orbit, so you don’t have to use Shift + MMB all the time, is that correct?
If yes, then you should know, there is no direct way of achieving this in SketchUp. The Shortcuts tab doesn’t offer mouse click changes. But luckily there is a workaround I can offer.
Have a look at [X-Mous Button Control] (X-Mouse Button Control) which I’ve been using for 3 years already, for some of Adobe’s products (Photoshop, Illustrator and Muse) to make their work-style similar to SketchUp.
I’ve tested the program to see if it’ll be able to do what you have in mind. I mapped the Middle Mouse Button to act as Shift+MMB when it’s pressed.
I can confirm it works and there’s no chance to accidentally Orbit the scene. 
One thing to remember though. Make sure you uncheck SketchUp from the list, whenever you want to revert back to normal work-style.