Bearfoil ...
Why is it thought that the a/c instantly started descending at first ACARS and took those 4 minutes to crash?
I'm not necessarily agreeing with that concept, but am guided by the very detailed analysis the BEA (obviously assisted by Airbus) have made and detailed of the debris recovered to date. Reference to the Interim Reports #1 & 2 will provide you with reasoning deduced for every one of the ACARS messages, and their sequencing.
The loss of signal from the a/c by the satellite in the period from 2:13:14z until a message sent at 2:13:45z has been explained by the BEA as either an a/c attitude event that caused antenna blanking or the total loss of both power sources. It is possible that the uncontrolled descent commenced during this period.
The Cabin Vertical Speed advisory has provided some indication of the FL passing through, when it was triggered. The actual time of this advisory (nominal timestamp 0214z) can only be guessed, but it was the last message sent/received by the aircraft. One fault message was effectively due to be sent at 02:15:14z, and though it was not received by the satellite due to "loss of signal" from the a/c at 02:14:28z, it is assumed that the aircraft's flight ended between 02:14:28z and 02:15:14z.
The BEA have carefully detailed the relationships between CAS, TAT and Mach, and also the upper/ lower boundaries at which disagreement will trigger PRIM/SEC faults. It is anyone's guess as to whether a mach critical or a low speed stall were the onset that lead to the end.
I don't believe there is anything further to be gained in this discussion until the evidence in the form of the DFDR/CVR is/are recovered and analyzed.
mm43