1989 Chevy Corsica cranks and dies

Maybe I'm barking up the wrong tree, but, when the vehicle stalls, leave the key in the on position and check primary voltage at coil and injectors.

I would try removing that fuse and running another wire from another power source with an inline fuse and run it to the alternator side of that fuse socket and see what happens. This is taking the ignition switch away from the alternator so we can see where this takes us.

Well?

Where did you take the power from?
Was it fused?

That's interesting. That means that the ignition switch is playing no part in the stalling and the alternator or the wiring is doing it completely on it's own. I might be tempted to try an alternator here. Your relying on some store's word that nothing is wrong and they could be wrong.

No, that is not true. The oil pressure sender is not designed as any kind of engine shut down. It is wired as a redundant power supply to the fuel pump so it can run without a fuel pump relay but cannot interrupt the power to the fuel pump if the relay is working.

Here is a little bit of info I found on the alternator wiring

The CS charging system has several sizes available, including the CS-121, CS-130 & CS-144 alternators. The number (121, 130 or 144) denotes the outside diameter in millimeters (mm) of the stator laminations.
CS generators use a new type regulator and a diode trio is not used. A delta stator, rectifier bridge, and rotor with slip rings and brushes are electronically similar to earlier generators. A conventional pulley and fan is used and, on the CS-130, an internal fan cools the slip ring end frame, rectifier bridge and regulator.
Unlike three wire generators, the CS generators may be used with only two connections, battery positive and an "L" terminal to the charge indicator bulb.
Use of the "P", "F", and "S" terminals is optional. The "P" terminal is connected to the stator, and may be connected externally to a tachometer or other device. The "F" terminal is connected internally to field positive, and may be used as a fault indicator. The "S" terminal may be connected externally to a voltage, such as battery voltage, to sense voltage to be controlled.
As on other charging systems, the charge indicator lights when the switch is closed, and goes out when the engine is running. If the charge indicator is on with the engine running, a charging system defect is indicated. For all kinds of defects, the indicator will be on with the engine running if system voltage is too high or too low. The regulator voltage setting varies with temperature, and limits voltage by controlling rotor field current.
This regulator switches rotor field current on and off at a fixed frequency of about 400 cycles per second. By varying the on-off time, correct average field current for proper system voltage control is obtained. At high speeds, the on time may be 10% and the off time 90%. At low speeds, with high electrical loads, on and off time may be 90% and 10%, respectively.
No periodic maintenance on the generator is required. The CS-121 and CS-130 are not repairable and may only be replaced as a single unit. The CS-144 is repairable.

I've taken you as far as I can. You have proven that when that alternator is supplied power is when the car dies so whatever is happened is at least related to the alternator wiring. The info I just gave you says that you can operate that system with only 2 of those wires so I would try isolating them. Something on that circuit is causing the problem. It may be after it comes back out of the alternator.