Battery run time question
According to some youtube videos, once up to temperature the Fridge
  used 27 Wh and 2 Amps in 4 Hours, averaging 6.75 Wh and 0.5 Amps  


The 0.5 A and the 6.75 Wh seems not consistent. the voltage has to be \$\frac{6.75 \text{Wh}}{ 0.5 \text{A}} = 13.5 \text{V} \$.  

The voltage at the output of the powerbank is \$ 12 \text{V} \$. 


consider only the Wh from the sources. Then amperage based on the available voltage at your powerbank output.   



  which result is correct?   


Calculation using Wh (Option 1). Option 2 yields wrong results because you are only dividing the amperage ignoring the voltage differences.