auto bypass

I need to add an ABV but there is no easy access to a return pipe, is it acceptable to bypass to the top of the coil (indirect cylinder) creating the bypass loop via the cylinder coil to the return?

The furthest radiator from my boiler is a 160cm x 50cm Type 22 in my living room. It has always been set as the bypass, but this isn't ideal. The living room can overheat, whilst the kitchen is cold. The obvious answer would be to set a lower temperature on the TRV in the living room, but this...