I'm sorta in the same boat, except for I'm not even explicitly trying to downgrade at the moment, just trying to get anything to work. My phone got dropped last weekend and cracked the back glass, subsequently would not hold a signal and reported a message that the phone needed to be restored.
Went from untethered 4.3.3 to stock 4.3.4 no problems, then same (hardware) problem, phone not able to keep a signal. Tried to do a restore again, stuck in endless Recovery mode now with error 1013. Host file has long since been modified to have the gs apple entries commented out, still no such luck.