Think of the act of starting. If you have an engine like a Modular with a stock pump that is relatively high volume/high pressure and it has a lot of things upstream of the pump that can bleed down, during a start-up event, the lack of pressure upstream of the media may allow for a brief moment of filter bypass until the engine becomes fully engulfed and the differential is reduced.
The delta-p across the filter media is only dependent on the flow volume going through it and the viscosity of the oil - doesn't matter what the pressure is on the outlet side of the filter. Doesn't matter if the oil galleries are completely full or completely empty of oil. And also, the oil volume hitting the filter is the same regardless if the galleries are full or empty because it's a positive displacement oil pump.
Picture a filter partially full of oil that suddenly experiences a massive volume of cold oil being forced against the media and the bypass with nothing upstream of it to provide any resistance (the engine is not yet fully engulfed).
The filter is experiencing the same oil volume flow it would always see (from the positive displacement oil pump), regardless if it is (and the oil galleries) bone dry, completely full or partially full. How is this filter seeing more some "massive" oil flow event just because it's not full of oil? The positive displacement oil pump is going to ensure the same volume goes through the system as long as the pump doesn't hit pressure relief, and in a start-up and engine idle condition the oil pump should easily remain below pressure relief point.
The operation of the bypass relies on a differential occurring in a fully pressurized system. That is, the pressure on both sides of the media and both sides of the bypass are close to equal as long as the media can flow enough oil.
The filter's bypass valve only operates due to the delta-p across the media. The system doesn't have to be "fully pressurized" for that to occur. If you mounted an oil filter on a bench and had the outlet just puke into a bucket on the floor (like a garden hose), and if you flowed X GPM of oil at viscosity Y through it, you would still see the same delta-p across the media. The delta-p across the media is only dependent on the oil flow volume, oil viscosity and media flow resistance. It doesn't care what the inlet or outlet pressure is ... just the difference between inlet and outlet.
When the media CANNOT flow enough oil, pressure on the OUTLET side is REDUCED enough that the valve opens.
Not sure what you mean by the pressure is reduced on the outlet side to make the bypass valve open (?). What happens is the pressure increases on the inlet side until the delta-p between inlet and the outlet is at or above the bypass valve setting - this is the only thing that opens the bypass valve. The pressure on the outlet side of the filter will always be lower than the inlet side, no matter what's going on in the oiling system - as long as there is some oil flowing.
If there is NO RESISTANCE upstream of the valve, it is going to open.
Not sure of your terminology use. To me when visualizing the flow path of the oil (or a stream of water), "Upstream" means "before" the valve (filter inlet) and "downstream" means "after" the valve (filter outlet). If you mean no pressure after (downstream) the filter/valve, then what you said is not true. If there was zero pressure downstream and 15 psi upstream, then the bypass would start to open if set to 15 psi. It's a 15 psi delta-p ... that's all that matters to the bypass valve.
More examples of a 15 psi delta-p would be as follow, and in all situations the bypass valve would start to open:
a) 15 psi inlet, 0 psi outlet (atm pressure)
b) 16 psi inlet, 1 psi outlet
c) 40 psi inlet, 25 psi outlet
d) 90 psi inlet, and 75 psi outlet
e) 200 psi inlet, 185 psi outlet
So that initial surge of volume that sees no resistance upstream of the filter on a cold start, if it creates 15psi of pressure inside the filter (which I'm sure is quite easy for you to imagine with cold oil) then the bypass is going to open because there is NOT 15psi of pressure on the OTHER side of that valve provided by the system UPSTREAM of the filter.
It would be a momentary event of course.
Well of course if there is enough delta-p across the filter (15 psi in your example), it will open the bypass valve - regardless of how the delta-p was created (key). But got to realize that you do not need pressure on the outlet side of the filter to make anything special happen with the bypass valve. All it cares about it the delta-p across it, regardless how it happens.
And yes, if there is a "surge" of cold viscous oil at start-up, then the bypass valve is probably going to open momentarily - regardless if the engine is "enveloped" or not.
Even if the engine was idling with hot oil at 200 deg F and you instantly smashed the throttle to the floor had hit red line in 1/2 second, that nearly instant surge of oil may cause a short momentary pressure spike that could pop the bypass open for a second or two. But if you slowly increased engine RPM from idle to red line with hot oil the bypass valve will probably not open at all.