A bypass is perfectly acceptable since the boiler should control according to the return temperature, that’s why even a shitty old Baxi that fell off the ark has a thermostat. The hot water circuit is independent of the central heating circuit so yes, you are wrong.
Bear to the forum, paging Bear.
The way combis work (and some older boilers) is that when the boiler heats up to make instant hot water it (the boiler) gets hot. You turn the tap off and the boiler still has lots of heat so it needs to dump that heat and uses the one mandatory open radiator to cool the boiler. The hot water circuit and the heating circuit use the same heat exchanger to get hot from the boiler. I think its called a calorifier. I have 4 properties all had new central heating installed since 1999 and all have an open radiator for this purpose.
I think trail-rat agrees with toys19
You will be dead ending your system when the heating goes off and the pump will continue to either pump water round for ages trying to cool it down or if it cant find a path over pressure
I think if you do not provide enough cooling capacity for your boiler through on of these rads, ie bypassing it as you reccomend, then you can damage the boiler.