If the plug is gone, then oil on the supply side to the main gallery can drain back past the filter through the passage. There is nothing to stop it from draining downward and leaving the pump and pickup 'dry' with no plug.
If you are referring to an accumulator having a cold start oil supply, that is done with a ball valve to store the pressurized oil when not running; in that state, the accumulator is essentially disconnected from the oiling system so has nothing to do with drainback, with or without the plug.
If you are referring to an accumulator working while the engine is running, then the oil from the accumulator should be blocked from flow back to the pump and pan by the filter's check valve (which is a simple rubber disc). If the plug was not there, then indeed the oil could flow back TO the pump gears as well as into the main passages from the accumulator; but the still spinning pump gears would keep it from going further, to the pan.
So the accumulator operation does not prove anything about drainback, or not, that I can see (expect that the filter's check valve is part of it working); if you see a point in the above that is wrong, then please make it clear where I am off-track. And, none of that is the normal drainback problem to which I was referring with the plug missing and the engine not running.
Yes, agreed on the passages, and pressurized oil on both sides. Not sure what you are trying to say; the pressures are different on each side of the plug due to drop in the filter element.
If anyone familiar with hydraulics wants to demonstrate that oil will flows in a circular path with the plug gone, then please do so on the basis of relative pressures at specific points. And if you want to challenge my numbers and my view of pressures at various points, please feel free do so.
