.... So you have no hope
of polishing your fuel unless you install an additional fuel line
(with a stop cock) before the injection pump

Yes, but.... A more convenient solution is to add a fuel pressure valve at
that stop cock point and always leave the cock open other then during a
crisis of some sort. I believe you can buy the little spring loaded pressure
valves which will open at 3psi for example.

When the engine is shut down but the electric pump is running, the valve will
automagically open and circulate the fuel back to the tank as desired as long
as the electric pump can make the 3psi or whatever you choose to use. When
the engine is running you'll still be getting at least the 3 psi at the input
to the injector pump, and I'm not sure why one would want more anyhow? Or if
one wants, your stop cock is still there to manually shut off the return
entirely.

Bill