Line-to-Line Voltage Reads 0V (or low, 15V at most) on Primary of Main Breaker (L-N both read ~120V)
I am not sure if this all adds up but maybe you lost a leg, and one leg is backfeeding the other through the water heater, you're seeing voltage drop from 118 to 105 across the water heater.  (I'd have guessed a bigger voltage drop but still...)  

With the water heater breaker off, do you lose power anywhere?