1. Erm, i was under the impression that you were using it as a heatpipe with gas to carry part of the heat in the interior . And that image at the top suggests that . The heatpipe principle involves some R12 (or similar) gas inside that has a very low boiling point.
With that image, and given the assumption its a heatpipe, it does look like and evaporator.
2. To create a passive heatsink it is possible, but i recommend trying with a longer pipe, longer fins, and (big AND) a very good case airflow. And maybe try with a solid copper center. Reduce the diameter, increasing the fin size.
Never the less, because you already have that one made, try to construct the ducting first.
3,4 You can make one like that out of a small pipe. Just carefully pick one with enought diameter to fit. You can use epoxy or even goop (silicone, or other glues, but they need to resist some heating) to attach it to the base. I've seen " T "shaped pvc pipes before, just like the ones used with the bong coolers, but finding one of the right dimensions must be hard. You can get a normal one, and cut it an and angle . Then you can glue them to the main pipe attached to the base.
Then 2 80mm fans or even 2 60mm fans should do the rest.
7. I've run my computer in passive mode for an afternoon. I just turned off the fans and put it on the window , because it was a windy day, it cooled it just fine
. Not pratical anyway.