Without trawling through the complete thread I thought that the RA failure had triggered 5 warnings to the crew. After all, wasn't this the reason they had configured to take all navigation information from the "B" system (for want of a better word). What was unknown was that the system would still take its height information from the right radar altimeter.
To play devil's advocate here, the crew had a very limited time to come to the realisation that something was wrong. For a considerable time after the aircraft had intercepted the ILS the speed was reducing to VREF so the thrust levers were at idle. When the crew did realise that the speed was decreasing below that speed the thrust levers were advanced (by the captain, I think) but he then took his hands off them and, of course, they retarded to idle.
Limited time, reconfiguring the aircraft, instrument conditions, a FO under some pressure and a trap lurking in the aircraft system..........the holes were lining up.