The Reservoir out port is open at the bottom. I can see hard plastic vacuum tubing that is not plugged -- is that the normal state?

Bottom of both yellow check valves are connected with "Y" hose that is fed from eng intake manifold. You mentioned Green line disconnected. Someone has modified the vac system. Sorry, I don't have complete info on #3 & #4 vac lines. I'll ask my inde tomorrow.
The only port that isn't used is on bottom of [rectangular] Change Over Valve. A Mighty Vac or similar tool will make testing easiest, but extra hose tapping off eng will do.
Remove battery and check condition of vac lines. Oddly mine cracked under the fan box. Remove right tire and screws top & back of plastic fender well. You'll see reservoir.


Would the diverter air flap pod account for the irregularly oscillating hissing I hear?

Yes, if the pod or feed hose is leaking air. You might be able to see it with the center vents removed. Otherwise, you'd have to remove dash [major major job] to access it. Therefore, I'd check everything else first.


I will test the controller tomorrow, pulling the air bag per these instructions I found searching threads yesterday:

Once you remove 10" bolt through bottom, us a flat head screw driver and gently pry on both sides of dash piece. It slides out very easily.


Your response posted while I was composing this one. Hopefully you're right on the AC juice. As an addendum: I just remembered the switches above the ACC. Center one looks like rotating circle. It's "Air Recirculation" switch that prevents outside air from entering cabin.