I doubt the ISIS (3-in-1 Standby Instrument) was fully lost: only the Airspeed part as the Attitude is a separate electric gyro powered by the DC Essential or Hot Battery Bus. ISIS Altitude is raw data from the standby static vents (does not go through Air Data Modules) - While pitot icing is likely I feel it is unlikely that the static vents iced up too. So ISIS attitude and ALT should have been reasonable. At the very least ISIS Attitude should have been good
These are also my thoughts.
At the very least altitude & attitude should have been available to our crew. Power & pitch would be the NNOP per the unreliable airspeed drill.
Has anyone ever ran this in the sim at altitude? I have set up for lower altitudes. It was no picnic. I can't imagine the challenges the real life situation would entail above FL300.