Hi Qavion
I'm having a bit of a problem with the Alternative Nav scenario. You won't be operating initially in Alternative Nav when you go to Standby power. For the first five minutes the L FMC will still be operating with all three IRUs and, therefore, the ND IRS(3) annunciation will be correct. However, at five minutes on battery power the centre IRU switches itself off and you are then left with the L FMC using the L IRU as its position input source - that should change the ND annunciation to IRS (L).
Just transitioning to Standby power won't put you in Alternative Nav - that only happens with a DOUBLE FMC failure.
Any way I'll get back to you on this - I want to see what happens tomorrow.