Really depends on how throughout the investigation is/what hardware survives the crash.
Depending on the surrounding circumstances of the incident, it would probably not even be considered a possibility because fatal car crashes are so common that "just another accident" is the most probable, and thus most usual answer.
That's also the reason why Michael Hastings death is so controversial to this day [0].
Nope. ECUs have something called a "fault memory" where the last n detected errors are stored, and that can be recovered. This is actually what workshops use for diagnosing a car.
This can give lots of information about what happened (faults could be "didn't receive message xyz" "sensor xyz gave signal out of tolerance").
But there is definitely no system trace for the communication - too many messages to really store them I guess.
Depending on the surrounding circumstances of the incident, it would probably not even be considered a possibility because fatal car crashes are so common that "just another accident" is the most probable, and thus most usual answer.
That's also the reason why Michael Hastings death is so controversial to this day [0].
[0] https://en.wikipedia.org/wiki/Michael_Hastings_(journalist)