"But for alignment, valid ADC data is needed...or else alignment will not complete."
Hmmm..... Tried to align the IRS's on a 747-400, yesterday, with all the ADC CB's pulled. After 10 minutes, apart from the expected missing displays such as V/S, altitude and speed, the IRS's appeared to align without any problems: ND Map displays with compass headings and PFD attitude displays seemed normal, there were no FMC scratchpad messages... and, as far as I could see, no additional EICAS msgs.
Are our manuals lying to us?
Rgds.
Q