I was simply correcting your authoritative statement: However, steering and brakes are mechanical systems in Hastings' 2013 Mercedes C250 (like almost every other car)

You are dismissing it as unlikely, because you keep assuming that some subsystems would work as designed or intended.

I'm not comfortable discussing details on this point, but "just use the brakes" might not have been an option had the malicious payload been properly designed. I have maybe a basic to mid level of experience reverse engineering automotive firmware, and it would not be hard for me to pull off. God only knows what a state actor could come up with.