You need to loop the return to the suction side of the lift pump. Looping to the output (pressure side) over rides the internal pressure regulator in the IP. There needs to be a pressure differential between the input & return side of the IP.
Plugging the banjo return will increase the temp of the diesel fuel when you're not running on WVO. The return to the tank is to keep the temp of the fuel down to maintain it's lubricity for the IP. It would be best to use a solenoid valve to redirect the return for each fuel.

__________________
82 300SD 202K
82 300SD 233K
83 300TD 340K
82 300TD 98k euro (parts on the hoof)
85 300TD 282K
83 300TD 197K