Remember that in this case, the "bottom" of the MacBook Pro with Retina Display is actually the keyboard/trackpad side. Injudicious application of heat -- whether through heat gun or hot plate -- could result in melting a few of the keyboard keys, or possibly damaging the trackpad.
The battery is removable, it's just very, very hard to do. The biggest issue is with the trackpad cable that's being routed under the battery, as seen here. One slip of the spudger / ruler / whatever you use to dislodge the battery could shear that cable, resulting in having to replace the trackpad as well.
The whole battery is held in place by a frame. You can dislodge the individual pieces partially, but you need to lift the whole enchilada out at the same time. Otherwise a.) at the very least you'll break the frame connecting them, or b.) you'll puncture one of the batteries and have a nasty, smelling, dangerous mess to clean up.
I speak from personal experience. :)