Problem is you've no safety reliefs without a bypass or an open rad. Tbh a bypass is best, why not just fit one ?
Or make another rad open if easier? The bypass is only a variable pressure sensitive connection between flow and return anyway.
How about the fact the system is vented to the header tank? Surely if it gets too hot, it will go into the header tank (overflowing if necessary) and in turn the tank will feed some cooler water back in? Still, the system is 20 years old and the only emergencies were a failed motorised valve and a shagged out programmer back up battery.......... Oh bollox, I know how pencils work......
It wont get too hot if you have flow. The boiler just shuts down. The only time the header tank sees flow is if theres a blockage, vent pipes in wrong place, or generally badly designed. If you dont know every pipe , just make small changes until you get it acceptable.
