ISIS Attitude information is independant of all three of the ADIRUS.
Regarding the ADIRU 1 + 3 inputs into the ISIS:
The ISIS has 2 possible power sources (ESS and Hot Bus)
When passing through 50kts the power source for the ISIS changes from the ESS to the Hot bus. How does it know when to do this? The airspeed information from ADIRU 1 + 3. Hence the reference to ADIRU 1 + 3 inputs.
This thread has gone interglactic in its amount of information. It reads like 80 people yelling at once. Good luck to you all and I hope we find the cause soon.