The message
from "Cicero" contains these words:

The return from the 10mm loop will feed into the total return from the
system at the manifold. The volume of water in a 10mm loop is really quite
small in comparison with the total volume in the system so it's not likely
to have a serious effect on the overall return temperature.

The volume of water in the loop is of no consequence. What matters is
the volume of water passing though the loop and at best this will be as
bad as an unbalanced radiator with both valves fully open and no way to
close them down short of putting a crimp in the pipe.

--
Roger