That would actually be a perfect scenario, imho. Mind you, if MF was in the driver's seat it may have caused some uncomfortable positioning? (Unless MF was sitting in the passenger seat, which personally, I'm not buying into a second person being involved yet.) But the car started apparently moving (if it was even the right car and people that the witness saw that day - putting too much weight into witness sightings has been problematic in some other cases) right away, so wouldn't have allowed time for MF to have strangled HB before driving (assuming still that MF is the only other person present).Maybe while Heidi was turned around buckling in MC, MF was in the front seat with the ligature leaning into the back? This thought involves another person driving and MF in the passenger seat though.
Thinking out loud....and it sounds too complicated - it would draw too much attention from other cars that could pass them.
But.. wherever they were going to, upon arrival, the same thing could have happened where say, MF pulls up really close to something on the driver's side, so that both she and HB have to get out of the car via the passenger side, MF opens the door for HB, HB turns her back to MF to get Margo out of the carseat, and wham, MF strangles her. The windows of MF's vehicle don't seem to be particularly darkly tinted, so would have had to have been at some kind of remote area, or MF was risking being caught on camera or by a witness. imo.