I proceeded to do a continuity test between the ground plane and Vss and *drum roll* theres a short between supply to ground somewhere!

I guess this is a mistype, Vss for logical chips SHOULD be ground, Vcc/Vdd should be 3V3/5V. Which chip has a Vss pin, most logical chips will have Vcc/Vdd and gnd.

reversed electrolytic? reversed 7805? reversed diode? other circuits on the board? PCB error.

To get close, you need to let the 1.6 Amps run and see if you can find a few millivolts disappearing somewhere. Often the best test instrument will be a finger, many such problems will produce heat at the problem area (I have scars on my fingers from such tests).

Let the Amps run, if a trace burns, you will know something and replacing a trace with a wire is no big deal.

Erik