Is this Thevenin equivalent circuit correct?
Yea you are doing it right. In the first case you have to add both \$ V_{AB} \$ voltages to get the net voltage across A and B, when both voltage sources are present.  With which you can draw the overall thevenin equivalent ckt. Second case is right too. Since there is no current through the R1||R2, the open circuit voltage \$ V_{BC} \$ will be nothing but voltage across R5 and R3 respectively in the last two circuits you have drawn.