Would a failed IRU be something that might be dispatched under MEL guidelines? If so, there should be a record. If something happened inflight to fail one IRU significantly before the loss of control, there should be a record on the CVR. It would be highly improbable to have a second IRU failure in close time proximity, unless there was some sort of common failure mode and with the IRU units being located close to each other (eg. an electrical fire, an environmental control failure or ??)
I've had a few attitude gyro failures in my lifetime and it has been an easy matter to find the indicator that still follows the control motions, but that probably assumes you have not already grossly upset the applecart.
The Captain knew he had problems, but why oh why did he just cry "help me, help me?" He should have known where to find good information and latched onto it. I suspect a panic reaction shut down his reasoning processes, Panic generally results from inadequate training-not knowing instinctively what to do next.
If the Pilot Monitoring was doing his job and had good attitude information in front of him, he should have taken over control and stabilized the situation when the aircraft departed its assigned altitude and particularly when the PF began to ask for help. Either PM was just along for the ride and didn't believe or understand his instruments, had bad information in front of him, or was looking at other things trying to understand what core problem was.
Like AF447, the crew in this accident seemed to have very weak altitude awareness. I suspect that the design of many PFD displays hampers altitude awareness under distracting and dynamic conditions.
For you folks who are currently flying the line, I have a question. Wouldn't you as pilot monitoring know if the other guy/gal is 'losing the bubble' and beginning to lose control? Otherwise, what are you monitoring?