I believe most brakes use a vacuum booster. Meaning you will likely get one or two assisted presses of the pedal before losing the assist. After that stopping the car becomes much harder. Try it next time you get in your car. Press the brakes a few times and see the difference in feel.
Have you tried at speed?
You are correct that they use a vacuum booster or similar, which is why when the car is turned off, you get a couple pumps. But, at speed, even with it turned off, there's not really any issue in being able to brake. Again, done it at 80 mph. Partly because it still translates force to the brakes, and partly because the strength you get from a clenched asshole and white knuckles vastly overcomes the vacuum boost assist.
In other words, it is definitely possible - as long as you can steer. That's the other thing about your car shutting off - power steering is gone. On a straight road, like I was on, fine - if you're in the middle of the mountains somewhere, well, I hope the aforementioned adrenaline helps you out, or you're likely fucked. A modern car without power steering is quite different from an older car without power steering.
How hard would it be to disable the device? It's just an electronic switch right? Should be able to remove it entirely.
I would have to look into it, but I assume the device 1) gets its power from the car and 2) transmits and receives data by cell. Which means it almost certainly has a heartbeat, "I'm here" transmitted every once in a while. Which means when you disconnect it entirely, whoever put the device on knows. That's just an assumption, I don't know for sure.
Can you remove it entirely? Absolutely. Consequences? Possibly.
If it legitimately shut down and locked everything up in the middle of the highway through no fault of her own (outside of not paying her bill on time), she would have had an excellent lawsuit, instead of "settling out of court for an undisclosed amount" mid-trial (ahem..."she drops charges and they won't counter-sue for legal fees").
To be fair, sometimes people settle out of court for huge sums of money; settling out of court is not an admission of guilt, legally nor practically. With that said, you are right, if it straight up shut her car off at speed, she has an
excellent lawsuit. Especially if, say, she claims she had children with her in the car... oh boy, that would really be something. GM is feeling the pain of cars shutting off at speed but they did it through incompetence, not on purpose.