My fdu 780 suddenly responds with a "reader error" when a known-good grand master key is swiped through the machine

Use a good code reader - it will show any trouble codes and can show a live view of most sensors.

The ignition switch may be the culprit as some have a chip that responds only to the right key which usually has a red tag. .if they don't marry up then car becomes immobilized.auto lock smith might be able to reprogram the key and ignition.you may get an auto shop to run a free diagnostic on the od11 input to see if there is an immobiliser error code stored In the ecu.good luck.

What prompted you to change the CPS? Was the issue existing before and you tried to solve it with a new CPS? I had a Jeep Grand Cherokee that kept giving me a bad CPS code. I replaced it and the problem didn't go away. Ended up being the computer.

I would take it back to where you had the work done. It sounds as if something wasn't tightened correctly.

Have you checked each individual door switch
for voltage? If there is voltage and a good ground try jumping the switch with a bypass wire to see if the window responds. It could be a bad master control switch which you can bypass to check to see if its working properly. Drivers door switch module. They are known to be a problem. Hope this helps.

I’m happy to help further over the phone at https://www.6ya.com/expert/gregory_8e6ba270263743ac

Why would you do 10 minute key trick ? Is the SECURITY light on steady ? Just a code reader ? Checks just Engine PCM codes ! Need a professional type scan tool to check codes in the BCM - body control module ! Don't know what and how much you know about these GM vehicles ? The ignition switch is a low voltage input to the BCM ! To determine power mode , off ,ACC , run , crank
On vehicles that have several control modules connected by serial data circuits, one module is the Power Mode Master, PMM. On this vehicle the PMM is the BCM. The PMM receives 3 discreet ignition switch signals to differentiate which power mode will be sent over the Class 2 Serial Data circuit.
To determine the correct power mode the PMM uses the following circuits:
• Ignition 0
• Ignition 1 voltage
• Ignition 3 voltage
Discrete Ignition Signals
Those modules that have discrete ignition signal inputs also remain in the state dictated by the last valid PMM message received on the serial data circuits. They then check the state of their discrete ignition input to determine the current valid state. If the discrete ignition input is active, battery positive voltage, the modules will fail-safe to the RUN power mode. If the discrete ignition input is not active, open or 0 voltage, the modules will fail-safe to OFF-AWAKE. In this state the modules are constantly checking for a change status message on the serial data circuits and can respond to both local inputs and serial data inputs from other modules on the vehicle . If you don't know anything about this your best bet , take it to a GM dealer . Without knowing what state the BCM is in or seeing from the ignition switch or if DTC'S - diagnostic trouble codes are stored , can't give you any sound advice . Would only be guessing !

What do you mean no brakes? pedal was stiff would not press down?, pedal went to floor? That needs to be checked. if there are no fluid leaks, I would suspect the master cylinder.

Check to see if you have a computer chip on your key. Chances are that the computer chip or the reader in the ignition keyhole is not reading it properly. If you leave the key on, engine off, for 15-20min. that security will reset and the light will go off. Turn key off and now try to start engine.

If your car hase a security chip in the ignition key I think you will find a broken wire going to the key reader in the ignition switch, they are known to break with age because they flex with each key cycle.