BOAC:
a) did the software or some mechanical effect prevent normal stall recovery action?
I'm not up on the mechanical factors, but I'd certainly be very surprised if there was a software fault involved - for two reasons:
Firstly, given the herculean level of pedantry that AI went to when designing these systems and the amount of ground and air testing they did, and sometimes continue to do on the prototype testbed, and also that the systems were designed to do everything in their power to avoid a stall situation, I'd be shocked if such a fault didn't show itself in testing very early on.
Secondly, the MidiBus has been hauling passengers around for 22 years now. Even if it hadn't shown up in testing, it would likely have shown up in line flying by now - certainly at a previous handover test like the one in this case.
The only automatics-related caveats I can think of relate to trying to force the nose down when TOGA is applied, and I'm pretty sure that the AI personnel present would have caught that kind of thing if it had happened. Also, I'm presuming that the FDR doesn't reflect this situation, as it would have been discussed to death on here by now if it had been.