For return protection the goal is to stop or divert flow when the boiler is running to cool (cold In your unique case we are trying to come up with the least problematic method. Put the sensor wherever it gets a sample of the boiler conditions. In your boiler the design may be fine with cool return if it blends quickly with all the volume inside and prevents condensing conditions. It will be obvious quickly in wood fired appliances as you will see condensation in the flue, and creosote forming quickly. Monitoring flue temperature is also and indicator, although it looks like you don't have low temperature issues there? In some cases manufacturers build sloppy, inefficient HX on purpose. A typical gas fired WH has a fairly inefficient HX design so it can handle low incoming water temperatures on a continuous basis, hence the high flue gas temperatures and low efficiency numbers.