"why don't the left and right ADC's feed their respective FMC's?"
You're not the only one wondering this, CW !
From my engineering training notes, I see that both L and R ADC's feed each FMC...and that the ADC that is used is dependent on AP/FD selection (as well as failures).
In your airline, when the Captain is the pilot flying, does he/she select the L A/P or F/D.. and when the F/O is flying, does he select the R A/P or R F/D? If so, the pilot flying would get to select the baro setting which goes to the active FMC....rather than, say, the FMC using an average of the two baro settings(not good if one pilot is incapacitated)
"Again, why do they both use the left IRU for this, and what altitude output is obtained from the IRU."
The altitude data (actually vertical velocity integrated) is the primary altitude source for the FMC (I suppose the ADCs provide long term correction). For the parameters you mention, the FMC's use the IRS corresponding to the autopilot in command. Is there a clue in there, somewhere? 8-)
Hopefully the info I have stated here is applicable to your aircraft.
Rgds.
Q.