I think SM killed Zahra in a fit of rage while dad was at work. I think dad came home from work as he said and Zahra was already in bed according to SM, (or locked in her room which seem was the norm and he did not check on her). SM had her body hidden in the car she drove already and when dad went to sleep removed her to his vehicle (suv) and disposed of her, drove back home and commenced her plan about the ransom note and gasoline poured in the suv in hopes it would catch fire from the brush fire.
She may not have expected the FD to be there so quick to put it out, or a 911 call placed so quickly. I don't know if the plan was to discover Zahra missing at that time or not................
I still do not think Dad knew his daughter was gone, and I believe she may try to pin it on him......jmho