After a re-read of the last report, it seems that the length of the search zone is principally deduced from the aircraft fuel endurance (figure 20, page 22 (27 of PDF)). The length on the arc between the northern and the southern points is about 2800 kms.
It would have been "fair" if the fuel remaining quantity presumed included in the last ACARS have been given in the report... And what is the accuracy of the fuel quantity measurement?
The BFOs study then came to reduce this length, but "The potential aircraft location, where the derived flight paths cross the 7th arc, is very sensitive to variations in BFO frequency. A 10 Hz variation in the fixed frequency bias can result in the derived flight path at the arc moving 1,000 km." (page 42, 47 of the PDF)...
The general impression, IMHO, is that there is some tautology in the report, like presuming the flight was on AP after the south turn ... and finally finding it was.
Is there any news about the possible faulty line of the table 6 (Downlink Doppler) page 58 (65 in the PDF)? If the values are the same in the three columns (as it could be presumed), then nothing can be deduced about a 5° latitude location change from the registered BFO!