I'm not sure quite how you read that, but there you go!
Yes. One reason to have the bypass! Alternatively, you could use a couple of relays and run the pumps and relays from the valve switches to give an OR for the boiler interlock.
An ancient gravity circulated boiler is unlikely to have pump overrun.
Perhaps, although a twin pump solution would be an easier fix, as it leaves the CH circuit untouched.
Christian.