Lonewolf and mm43;
From BEA's no.2, para. 1.6.11.4 (again):
In alternate or direct law, the angle-of-attack protections are no longer available but a stall warning is triggered when the greatest of the valid angle-of-attack values exceeds a certain threshold. In clean confi guration, this threshold depends, in particular, on the Mach value in such a way that it decreases when the Mach increases. It is the highest of the valid Mach values that is used to determine the threshold. If none of the three Mach values is valid, a Mach value close to zero is used. For example, it is of the order of 10° at Mach 0.3 and of 4° at Mach 0.8.
Here the stall warning was 6°, not 10° or 4°, corresponding to the actual Mach number of about 0.68. So one of the Mach values was valid. The 'polled' speed is what the PRIM's select to use. If all three values are valid, it is the middle (median) of the three values. If one ADR has been rejected, it is the average (mean) of the two remaining values. If the difference between the two remaining values exceeds a tolerance for a certain time, you get ADR DISAGREE. That's about the limit of my knowledge.
Furthermore it is noteworthy, since stall warning and stall do not occur in normal law, that the master PRIM was functioning in Alternate (2) law.