"Another thought I had was to start the system cold while manually closing both ports to mimic pump overrun, then adjust the ABV until I feel heat on the return."
That's basically how you do it, it doesn't matter though what the PP setpoint is which you set to satisfy your heating load first, the fact is that the by pass volume will increase when the zone valves open because the pump head increases, this is the opposite to the nomal constant curve ( or the old 3 speed pumps) where the maximum pump head is with the zone valves closed and it then falls with increasing flow so the ABV closes almost as soon as the zone vales open.
Its very normal not to have pump overrun with a oil fired boiler (mine doesn't) as the zone valves open. OF boiler's HX acts as a buffer since it holds 25 to 30 litres of water, which will soak up the residual combustion chamber heat, the hi temp o/limit stat opens at 110/113C which is a long way from the 55C/75C boiler stat SP. I have measured my temp at a max of 90/95C after shutdown with a boiler SP of 70C.
You might also notice how the burner can take up to 5 minutes or so to refire after cycling off, that's because its using up the stored heat in the Hx until the temperature in the top falls to boiler SP-8to10C.