I finally got around to precisely measuring the different EC to MITERS routes on Google Earth. I wasn't sure how the path through the Swapfest parking lot compared to the others, but this confirms its about the same distance. As for the numbers:
White = 693 meters (the absolute shortest distance to MITERS from EC)
Red w/shortcut = 848 m (includes the sidewalk currently blocked by construction on building 35)
Red w/o shortcut = 856 m
Blue = 858 m
Purple = 858 m
Orange = 964 m
So really, the shortest path is past Building 13 and along Mass Ave with the diagonal sidewalk towards the intersection of Mass Ave. and Vassar (which only saves 10 meters (+/- measurement error)).
The red path is also probably the fastest because of the straight stretch past Building 13. However, I've gotten a little bored of the same route, and have now switched to the purple path (even though it's 2-10 meters longer).
Just for kicks, here's the same route placed over MIT the year after I was born.