Thanks for the link. I found a helpful bit about the test mode connectors I didn't see when I read that page yesterday. I spent a good couple hours reading around different Subaru forums but didn't find much.
I still can't figure out which solenoid is the "vent control solenoid valve" the CEL refers to.
Last night I dropped the charcoal canister down and removed one of the solenoids. Nothing seemed obviously wrong but I ran out of daylight so I put it back together.
Today I'm gonna completely remove the charcoal canister and check it out. I'm also gonna remove and inspect every hose back there to check for blockage.
Not sure what else to do besides that....