I uncommented the dual extrusion code in my config.g, and the extruder started working.
Bafflingly, I tried commenting out the dual extrusion code again to see if the issue came back, and it didn't. So I'm running the same config.g as above, with no other hardware or software modifications and the extruder works fine. I'd tried multiple config modifications before to no avail, and rebooted it many times prior to this. So I have no idea what the issue was, or why it disappeared, but it's gone now.