I agree. The phone would have been MF's biggest problem. She must have got control of it very quickly I think, before Heidi would have become alarmed. I wonder how she ensured that Heidi wouldn't tell anyone she was seeing MF between their assumed conversation at 8.30 ish and 9.05am. Perhaps she didn't tell her she was in the area, but told Heidi to check downstairs for a surprise delivery in the mail, or something.
That timing can't have been coincidence, that Heidi walks out just as MF pulls up.
It does sound to me as if another priority of MF's would have been to not give Heidi time to let anyone know she was seeing MF. But the greeting and entry to the car doesn't sound to me like Heidi was surprised or stopped to think, it sounds like it was quick to have happened all while this witness was driving/on the move and still having them in her sight.
Just now gotta think how the purse got in the car, but keys and phone in her hands. Does Heidi have to produce her apartment key as proof she is a resident to collect her mail, or to open a mail box?