1. To get the code from the check engine light you need to short the connector under the carpet at the ecu, it is the connector that is plugged in on top to a holder. Once shorted (with a paper clip or something like it), turn the key to the ACC position and the check engine light will flash the code number.

2. As noted above the cabin filter is located on the blower assembly behind the glove box.

3. To keep the factory alarm from turning on you can short the connector by the hood release, that way you fool the car into thinking the hood is up and it will not arm the factory alarm.

All the above is really well described also on www.acura-legend.com where there are more legend owners.