All good points. Seems like you could get around it by using these other-car communications as noisy signals and weighting evidence against the world as the car sees it, e.g., if you get a spoofed emergency brake advisory, and the car's own percepts suggest there's no reason to brake, the resultant action may be to not brake. The signal from the other car[s] becomes just another feature.
Considering the millions of miles driven each day, even if the networked signal wasn't heavily weighted, a spoofed emergency brake advisor signal could still do significant damage.
