i think you should have a return line that goes back to the veggie tank. The manufacturer put return lines into the equation for a reason. Besides, returning wvo to the tank helps to keep the tank hot (returning oil is always hotter than oil going to the engine) And return lines get rid of air automatically. If you run wvo, there will be times you get air in the system. Having to manually find a way to bleed air out of the system would be a drag.
Gravity feed is the best option in my opinion. Cheap, and simple. No lift pumps or electricity needed.
No, you do not need a pump on the return lines. Plenty of pressure there.
I do like to install a pump into the diesel fuel line feeding the engine. I find this most helpful for re-priming the engine after running out of fuel. Alternatively you can add one of these into the diesel fuel line: They are only a few bucks at wal=mart in the summer months.
However, the electric pump is preferred because if you run out of wvo or filters become clogged etc when you switch back to diesel while driving the engine will not prime itself without some help. If you switch on the electric fuel pump then most times the bus will start right up and you can drive away which is pretty handy when you are blocking traffic on a major road.