Your Drelloscop 3018 isn't really designed to be triggered by a camera connected via a pc port. A pc port is not an active control system. Neither is the center pin on your hot shoe. Neither sends any energized signal from the camera to whatever is connected to the camera. Rather, they merely close an open loop circuit that allows whatever electrical signal/voltage is being supplied by the connected flash unit to return to the flash unit and trigger the flash. If your connection to the Drelloscop 3018 is not supplying any "trigger" voltage then completing the circuit when you press the shutter will not do anything except connect two wires, neither of which have any voltage/signal flowing through them.
My suspicion is that when you are using the Hahnel RF trigger there is enough stray voltage bleeding into the output from the receiver to trigger your Drelloscop which is probably very sensitive to any electrical signal. Similarly, there may be a contact on the Olympus hot-shoe extension that is touching one of the control contacts of your Nikon's hot shoe that is picking up just enough of a signal from the camera's flash data communication chip to kick the strobe on. How that micro-voltage is getting through the AS-15 is kind of a mystery, though.
Note: You should be very careful about hooking electrical devices to your camera's PC port or hot shoe until you have verified that the camera can handle the potential output voltage of the other device. Many old flashes have output voltages greater than 250V while many modern DSLRs can only tolerate voltage in a much lower range. Some can be as low as 6V! Hooking a 400V flash up to a 6V camera is a good way to fry the camera's flash control circuitry at a minimum and potentially the entire camera's electronic system.