If I connect with GPS only should I still be able to use the revo's on-board mag for INS13?
Yes, but it is unlikely that on-board mag will give error-free measurements usually on quads <= 300mm this is hard.
Depends on your flight controller location, if it's far from power lines, and twisted wires can help a bit. Also ESC are switching components and they work like electromagnets.
Usually you can't shield from magnetic field, unless you use 3-4 meters concrete wall, like they do at Large Hydron Colider
what you can do best is redirect magnetic field by twisting wires to make it less severe. Any "shielding" around magnetometer is also bad idea, as it may distort static magnetic field and confuse magnetometer.
The best is to have mag on a pole and calibrated outdoors over OPlink far from metallic elements, but you can try of course.