From what I have heard about the data analysis, I would think you would get the same doppler effect and resulting positional data from both a southern route and a mirror northern route toward India and Pakistan. The satellite did not get directional data, so how would they eliminate the mirror image route?
You are correct that the satellite cannot ascertain a direction, unlike human ears which can because we listen in stereo.
The northern mirror image route would take the plane over areas that would be covered by other sources (like Indian military radar). The lack of a signal from the northern route leaves only the southern route.
However, if the plane had changed speed, it may have been possible for the plane to go on a northern path that didn’t cross airspace that wasn’t monitored. I don’t know those details.
I read somewhere Inmarsat dismissed the norther route as “too trafficked”. Yeah.