Running a return is no problem at all with a suitable regulator.

You say it the fuel overflowed...was this at 2.5psi or is there no gauge after setup ?

It could either be excessive pressure, or simply the needle valve sticking due to dirt or other.

But if the needle valves are sealing, they should have no issues with a dead and and 2.5psi, most should handle 4-5psi.

The fact it's only one carb filling up, again, investigate float levels and needle valve condition.