You definitely need a one way valve on the return line. If the line is coming out of the header tank, it is being pressurized from the column of fuel in the wing tanks and it is not working as a fuel return, but rather another source of fuel to pressurize the carbs.
To me, it sounds like you have too much fuel pressure. I am currently working thru a similar situation on my Rotec engine with a new TBI that required a fuel return. I have 2 electrical pumps, but when either is on, fuel leaks out the TBI at idle. When I shut both off and just use tank head pressure, it works great. I don’t like the idea of no fuel pump, but it flys in all aspects of flight without one (ie low fuel, full throttle, max climb angle, etc). I’m considering going with a lower pressure fuel pump for take off and landing to see if that prevents the problems I am seeing..... I think I too am over pressurizing the system. (Also my fuel flow goes way up when the fuel pump is on as I am returning all the excess unused fuel back to the tanks). When I shut the fuel pumps off, I see fuel flow and fuel burns about where they used to be.