There was a tech bulletin on replacing the cam/crank sensors in sets. You determined which ones needed to be updated based on the color and date stamp. Subaru used to replace these free of charge to the customer, but the "kit" is no longer available since they quit doing that. The sensor cost approx $100 each wholesale.
MAF, if you have a scan tool, should show 3 to 3.8 g/s at normal operating temperature idle. Pin testing the MAF is only accurate at specific temperatures so that's not an option unless you can control the temp of the MAF for the period you are testing.
Knock sensors will set the MIL for knock sensor circuit when they fail. You can check it for cracks, corrossion, broken wire.
Did you get a parts list from the parts store that read the code from your ECM?