NSEU,
Methinks the "IRU LEFT" QRH is wrong. My notes state...
BSCU inputs NECESSARY for BGS arming logic ---
1-- Wing gear wheel speed <15kts
2-- Both BSCU air/grd inputs indicate grd mode >10secs
3-- Left OR right IRU grd speed signal <40kts
As far as the BSCU databusses, would they not both be there in case of IRU failure as you have indicated by Capt"s or F/O"s "SELECTED" IRU? Open for correction...