IF all TRV's are off, then the bypass loop will be the only thing
getting heated. Basically thet is so small as to be almost irrelevant.

That depends on what your bypass is. A tiny bypass internal to the boiler is
likely to be small. However, a permanent bypass through a bathroom radiator
is likely to be much larger.

I'm still not sure why you are so resistant to having a ten quid flow switch
in the system. Over the lifetime of the system, it will definitely save many
times that in gas and isn't much of a risk.

Christian.