The Doppler tracing and tracking carried out by INMARSAT on the SATCOM signals plus validation testing against other aircraft would put the aircraft in the area being searched. It is not possible for the aircraft to have gone North and still provided the same Doppler signals. These calculations have also been checked by a considerable number of mathematicians.
There _is_ some discussion on the actual fuel burn that the aircraft could have made as it is thought to have made at least one low pass before flying South this could lead to an along track error that would have the aircraft crash/ditching site further North.