Thevenin voltage calculation
\$V_B−V_A =−14V = V_{AB}\$


This is wrong.... 

\$V_B−V_A  =  V_{BA}\$ , not \$V_{AB}\$  

When we use the notation \$V_{AB} \$, it means the potential difference/voltage between A and B. 

We expand it as \$V_A−V_B\$, not \$V_B−V_A\$. 

And
\$V_{AB} = - V_{BA}\$