You're right that the bypass valve in the filter is activated by differential pressure. When the oil doesn't flow well and cannot pass through the filter media somewhat easily, pressure is building and the bypass valve is forced open to allow oil through the filter unfiltered. This happens more often than you think. Like at cold startup and it doesn't even need to be super cold ambient temperature.
Just because a filter is on the pressure side (like the 993 filter housing that replaces the old style engine-mounted oil cooler) doesn't necessarily mean the filter is receiving more pressure. Peak system pressure that you see on your oil pressure gauge is dictated not necessarily by the pump itself, but ultimately by the spring valves in the engine case.
Actually the scavenge side of the 911 pump is much more robust. The volume of oil delivered by the pump is determined by the physical size/length of the pump. Look at any of the pumps (not just the big boys like the GT3 or the 930 pump) and you'll see the size of the pump between the venturi/suction pickup and the discharge port is MUCH bigger than the size of the pump where the oil is delivered to the bearings.
The pressure on the scavenge side can be REALLY high especially when cold. That oil pump is sucking up and pushing out a larger amount of oil from the engine sump and it has to go somewhere. The external thermostat has a built in bypass valve and apparently that is an important thing to have in terms of protecting the scavenge side of the system from very high pressure.
http://forums.pelicanparts.com/911-engine-rebuilding-forum/695694-creating-crankcase-vacuum.html#post6936128
Regarding how the tank drain plug can have major debris on it, I agree that's very concerning. When you look at how the tank takes oil in, that debris has traveled through a lot of places you don't want it to go before it ends up on the bottom of the tank. For that rocker debris to end up in the bottom of the tank, it has to fall via gravity from the cam housings, travel through the oil return tubes, fall into the engine case sump, get sucked up by the oil pump scavenge side, then is pumped out to the external thermostat and finally to the oil filter on the tank. Whatever the filter doesn't filter (because the particles are too small or the filter is in bypass mode) then gets dumped right into the tank where it cascades onto onto a coarse screen. I don't think that screen is any finer than the oil pump pickup screen. So it drops to the bottom of the tank and the last line of defense, which is weak, is the drain plug magnet. So that means any debris not stuck to the magnet is ultimately sucked out of the tank by the pressure side of the pump and therefore goes into the oil pump to do its damage again.
http://forums.pelicanparts.com/porsche-911-technical-forum/982980-diy-oil-tank-cleaning-possible.html#post9873429
The above described route of oil is based on the older engine oil system. The later oil system like the 964 has the oil go through the filter housing (which is separate from the tank) before it goes to the thermostat and back into the tank. But even with the separate oil filter, that doesn't change the outcome. The oil filter still doesn't catch the debris if the filter is bypassing it.