I think you are thinking about this to much. The hose with the valve hooks to the top of the bag and either hose on the floor hooks to the bottom, it really is that simple.
With only one drain you don't get the quicker draining of the bag to the tank as its all gravity doing the work and you will only have one but it only affects drain time. You will probably have to drain, turn it off, wait for the bag to drain into the hard tank and then drain again. I have heard some users mention this has to be done 2 or 3 times to get all the water out.
As for the no vent just order one of these,
http://www.wakemakers.com/fly-high-pro- ... -w749.html , that should eliminate the venting issue.
Let me know if you have any other questions.
Jason