Joe,

I believe the people who have installed the Facet pump plumb it in series - that was what I did placing the pump under the center console.

I am not aware of anyone using a return line because of installing the electric pump. A return line is used on the newer Rotax engines in conjunction with a calibrated orifice to relieve pressure between the mechanical pump and the carbs to help prevent flooding in hot soak conditions after shutdown.

The basic Kitfox fuel system is designed without an electric backup pump and a case can be made for either leaving it out per the original Kitfox plan or installing one. The electric pump can't solve every possible failure mode of the mechanical pump. It's a builder's discretion item. I do not normally run with the pump on; but, use it on startup to help with a quick start; and as a standby backup for the mechanical pump.