I have been giving this topic a lot of thought. It seems to me that there are a lot of short comings from both the greasecar.com (returned) and the greasel.com (looped) kit.
One of the things I have been trying to come up with a better design for is the looped / returned line topic you have asked about.

The looped design is the best overall because if you don’t loop the line then you have to purge the WVO out of the return line with diesel. If you don’t you risk the WVO solidifying and clogging the lines. This is an eventual problem in the works.

What I like about the looped system is the quickness of the purge. If you have the 3 way valve as close to the IP as possible all you have to do is purge the WVO out of that inch or two of line and what is left in the IP.

Now what I am thinking is the absolute best way to do this is to have 2 x 3-way valves and a combination of the return and looped system.

My reasoning for keeping the return line is a quicker heat up time for the WVO tank and more highly filtered WVO.
I figure if you have it setup where the WVO has a heated supply and return lines it will constantly pump heated WVO back to the fuel tank. This should cut down on tank heating time. Also the fuel will have passed through the filter several times instead of just one time like in the looped system. This is especially useful if you are using a water separator.

The other 3-way valve would be to switch the return line (from the IP) to a loop when running on diesel. This way you still get your quick purge.

I hope this makes since.