In your scenario, while the Doppler shift may also have been negative the time delay and angle would have remained constant.
It's seem there are only "delta" values, not negative ones: if negative was possible, the path towards the sat (from "disappearance" to west of Malacca Straits) have to be also negative.
For example, if after about 5 hours of flight the pilot would simply turn 180 degrees, fly that track for half an hour, turn 180 degrees and repeat untill fuel exhausted... wouldn't the relative angle to the satellite still be about the same?
The sat data give a speed (Doppler) but also a distance (in and out signal). So, with your guess, the last "arc" couldn't be 40° but a smaller value.
All this from I have understood...I